The people of Israel were awaiting their salvation, they thought that this man might be it. He had performed so many miracles that they began to believe that he was who he claimed to be. He feed thousands with a loaf of bread and a few fish and he even raised the dead. How could someone who wasn’t from God do that? But he wasn’t doing the things that he was supposed to do, he wasn’t taking control of Jerusalem. When he entered the city, he didn’t set up his throne, he cleared the temple instead. He should have been preparing the people to take control of Jerusalem, instead of condemning Rome for it’s oppression, he condemned the scribes and the Pharisees instead — he called them hypocrites and white washed tombs. This didn’t sound like the king they were expecting.
So, they condemned him to death because he didn’t meet their expectations. He wasn’t what they wanted. They tauted him on the cross, hurled insults at him:

Matthew 27:39 And those who passed by derided him, wagging their heads 40 and saying, “You who would destroy the temple and rebuild it in three days, save yourself! If you are the Son of God, come down from the cross.” 41 So also the chief priests, with the scribes and elders, mocked him, saying, 42 “He saved others; he cannot save himself. He is the King of Israel; let him come down now from the cross, and we will believe in him. 43 He trusts in God; let God deliver him now, if he desires him. For he said, ‘I am the Son of God.'” 44 And the robbers who were crucified with him also reviled him in the same way.
Luke 23:35 And the people stood by, watching, but the rulers scoffed at him, saying, “He saved others; let him save himself, if he is the Christ of God, his Chosen One!”
Luke 23:39 One of the criminals who were hanged railed at him, saying, “Are you not the Christ? Save yourself and us!”

Jesus knew they didn’t understand:

Luke 23:34 And Jesus said, “Father, forgive them, for they know not what they do.” And they cast lots to divide his garments.

But the suffering of the Servant of God was clearly revealed in the prophet Isaiah:

Isaiah 53:1 Who has believed what they heard from us? And to whom has the arm of the LORD been revealed? For he grew up before him like a young plant, and like a root out of dry ground; he had no form or majesty that we should look at him, and no beauty that we should desire him. 3 He was despised and rejected by men; a man of sorrows, and acquainted with grief; and as one from whom men hide their faces he was despised, and we esteemed him not. 4 Surely he has borne our griefs and carried our sorrows; yet we esteemed him stricken, smitten by God, and afflicted. 5 But he was wounded for our transgressions; he was crushed for our iniquities; upon him was the chastisement that brought us peace, and with his stripes we are healed. 6 All we like sheep have gone astray; we have turned every one to his own way; and the LORD has laid on him the iniquity of us all. 7 He was oppressed, and he was afflicted, yet he opened not his mouth; like a lamb that is led to the slaughter, and like a sheep that before its shearers is silent, so he opened not his mouth. 8 By oppression and judgment he was taken away; and as for his generation, who considered that he was cut off out of the land of the living, stricken for the transgression of my people? 9 And they made his grave with the wicked and with a rich man in his death, although he had done no violence, and there was no deceit in his mouth. 10 Yet it was the will of the LORD to crush him; he has put him to grief; when his soul makes an offering for sin, he shall see his offspring; he shall prolong his days; the will of the LORD shall prosper in his hand. 11 Out of the anguish of his soul he shall see and be satisfied; by his knowledge shall the righteous one, my servant, make many to be accounted righteous, and he shall bear their iniquities. 12 Therefore I will divide him a portion with the many, and he shall divide the spoil with the strong, because he poured out his soul to death and was numbered with the transgressors; yet he bore the sin of many, and makes intercession for the transgressors.

They didn’t understand that they had just crucified their Messiah. They didn’t understand that he came to do this very thing, to die for the sins of his people. They were expecting to live in a kingdom of peace and righteousness but how could they expect to that when they had been unable to do so throughout their history? They thought the kingdom would be governed by the Law but how would they keep it, since they have never been able to keep it before. God gives them the answer through the prophet Jeremiah:

Jeremiah 31:1 “At that time, declares the LORD, I will be the God of all the clans of Israel, and they shall be my people.” 2 Thus says the LORD: “The people who survived the sword found grace in the wilderness; when Israel sought for rest, 3 the LORD appeared to him from far away. I have loved you with an everlasting love; therefore I have continued my faithfulness to you. 4 Again I will build you, and you shall be built, O virgin Israel! Again you shall adorn yourself with tambourines and shall go forth in the dance of the merrymakers. 5 Again you shall plant vineyards on the mountains of Samaria; the planters shall plant and shall enjoy the fruit. 6 For there shall be a day when watchmen will call in the hill country of Ephraim: ‘Arise, and let us go up to Zion, to the LORD our God.'” 7 For thus says the LORD: “Sing aloud with gladness for Jacob, and raise shouts for the chief of the nations; proclaim, give praise, and say, ‘O LORD, save your people, the remnant of Israel.’ 8 Behold, I will bring them from the north country and gather them from the farthest parts of the earth, among them the blind and the lame, the pregnant woman and her who is in labor, together; a great company, they shall return here. 9 With weeping they shall come, and with pleas for mercy I will lead them back, I will make them walk by brooks of water, in a straight path in which they shall not stumble, for I am a father to Israel, and Ephraim is my firstborn. 10 “Hear the word of the LORD, O nations, and declare it in the coastlands far away; say, ‘He who scattered Israel will gather him, and will keep him as a shepherd keeps his flock.’ 11 For the LORD has ransomed Jacob and has redeemed him from hands too strong for him. 12 They shall come and sing aloud on the height of Zion, and they shall be radiant over the goodness of the LORD, over the grain, the wine, and the oil, and over the young of the flock and the herd; their life shall be like a watered garden, and they shall languish no more. 13 Then shall the young women rejoice in the dance, and the young men and the old shall be merry. I will turn their mourning into joy; I will comfort them, and give them gladness for sorrow. 14 I will feast the soul of the priests with abundance, and my people shall be satisfied with my goodness, declares the LORD.” 15 Thus says the LORD: “A voice is heard in Ramah, lamentation and bitter weeping. Rachel is weeping for her children; she refuses to be comforted for her children, because they are no more.” 16 Thus says the LORD: “Keep your voice from weeping, and your eyes from tears, for there is a reward for your work, declares the LORD, and they shall come back from the land of the enemy. 17 There is hope for your future, declares the LORD, and your children shall come back to their own country. 18 I have heard Ephraim grieving, ‘You have disciplined me, and I was disciplined, like an untrained calf; bring me back that I may be restored, for you are the LORD my God. 19 For after I had turned away, I relented, and after I was instructed, I slapped my thigh; I was ashamed, and I was confounded, because I bore the disgrace of my youth.’ 20 Is Ephraim my dear son? Is he my darling child? For as often as I speak against him, I do remember him still. Therefore my heart yearns for him; I will surely have mercy on him, declares the LORD. 21 “Set up road markers for yourself; make yourself guideposts; consider well the highway, the road by which you went. Return, O virgin Israel, return to these your cities. 22 How long will you waver, O faithless daughter? For the LORD has created a new thing on the earth: a woman encircles a man.” 23 Thus says the LORD of hosts, the God of Israel: “Once more they shall use these words in the land of Judah and in its cities, when I restore their fortunes: “‘ The LORD bless you, O habitation of righteousness, O holy hill!’ 24 And Judah and all its cities shall dwell there together, and the farmers and those who wander with their flocks. 25 For I will satisfy the weary soul, and every languishing soul I will replenish.” 26 At this I awoke and looked, and my sleep was pleasant to me. 27 “Behold, the days are coming, declares the LORD, when I will sow the house of Israel and the house of Judah with the seed of man and the seed of beast. 28 And it shall come to pass that as I have watched over them to pluck up and break down, to overthrow, destroy, and bring harm, so I will watch over them to build and to plant, declares the LORD. 29 In those days they shall no longer say: “‘ The fathers have eaten sour grapes, and the children’s teeth are set on edge.’ 30 But everyone shall die for his own sin. Each man who eats sour grapes, his teeth shall be set on edge. 31 “Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah, 32 not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the LORD. 33 But this is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people. 34 And no longer shall each one teach his neighbor and each his brother, saying, ‘Know the LORD,’ for they shall all know me, from the least of them to the greatest, declares the LORD. For I will forgive their iniquity, and I will remember their sin no more.”

Before they went into the promised land, Moses told them to remember the commandments he had given them, they were to bind them on their hand and forehead:

Deuteronomy 6:1 “Now this is the commandment, the statutes and the rules that the LORD your God commanded me to teach you, that you may do them in the land to which you are going over, to possess it, 2 that you may fear the LORD your God, you and your son and your son’s son, by keeping all his statutes and his commandments, which I command you, all the days of your life, and that your days may be long. 3 Hear therefore, O Israel, and be careful to do them, that it may go well with you, and that you may multiply greatly, as the LORD, the God of your fathers, has promised you, in a land flowing with milk and honey. 4 “Hear, O Israel: The LORD our God, the LORD is one. 5 You shall love the LORD your God with all your heart and with all your soul and with all your might. 6 And these words that I command you today shall be on your heart. 7 You shall teach them diligently to your children, and shall talk of them when you sit in your house, and when you walk by the way, and when you lie down, and when you rise. 8 You shall bind them as a sign on your hand, and they shall be as frontlets between your eyes. 9 You shall write them on the doorposts of your house and on your gates.

But in Jeremiah we see that God will put his commands in their heart. They will no longer be external but internal, a part of them. They will keep the commandments because they will no longer be external rules that they have to remember but part of who they are. We see this thought developed further in Ezekiel:

Ezekiel 11:19 And I will give them one heart, and a new spirit I will put within them. I will remove the heart of stone from their flesh and give them a heart of flesh, 20 that they may walk in my statutes and keep my rules and obey them. And they shall be my people, and I will be their God.
Ezekiel 36:22 “Therefore say to the house of Israel, Thus says the Lord GOD: It is not for your sake, O house of Israel, that I am about to act, but for the sake of my holy name, which you have profaned among the nations to which you came. 23 And I will vindicate the holiness of my great name, which has been profaned among the nations, and which you have profaned among them. And the nations will know that I am the LORD, declares the Lord GOD, when through you I vindicate my holiness before their eyes. 24 I will take you from the nations and gather you from all the countries and bring you into your own land. 25 I will sprinkle clean water on you, and you shall be clean from all your uncleannesses, and from all your idols I will cleanse you. 26 And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. 27 And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules. 28 You shall dwell in the land that I gave to your fathers, and you shall be my people, and I will be your God. 29 And I will deliver you from all your uncleannesses. And I will summon the grain and make it abundant and lay no famine upon you.

How would they keep the law? God would not only put the law in their hearts but would fill them with his Spirit who would cause them to walk in his statutes. He would guide them to righteousness. But what about the sins that they had already committed? God says that he will purify them, that he would cleanse them of their sins. In the Old Testament the only way to be cleansed was the shedding of blood:

Hebrews 9:18 Therefore not even the first covenant was inaugurated without blood. 19 For when every commandment of the law had been declared by Moses to all the people, he took the blood of calves and goats, with water and scarlet wool and hyssop, and sprinkled both the book itself and all the people, 20 saying, “This is the blood of the covenant that God commanded for you.” 21 And in the same way he sprinkled with the blood both the tent and all the vessels used in worship. 22 Indeed, under the law almost everything is purified with blood, and without the shedding of blood there is no forgiveness of sins.

They themselves knew that the blood of animals was not enough because they had to repeatedly shed the blood of animals to atone for their sins:

Hebrews 10:1 For since the law has but a shadow of the good things to come instead of the true form of these realities, it can never, by the same sacrifices that are continually offered every year, make perfect those who draw near. 2 Otherwise, would they not have ceased to be offered, since the worshipers, having once been cleansed, would no longer have any consciousness of sin? 3 But in these sacrifices there is a reminder of sin every year. 4 For it is impossible for the blood of bulls and goats to take away sins. 5 Consequently, when Christ came into the world, he said, “Sacrifices and offerings you have not desired, but a body have you prepared for me; 6 in burnt offerings and sin offerings you have taken no pleasure. 7 Then I said, ‘Behold, I have come to do your will, O God, as it is written of me in the scroll of the book.'” 8 When he said above, “You have neither desired nor taken pleasure in sacrifices and offerings and burnt offerings and sin offerings” (these are offered according to the law), 9 then he added, “Behold, I have come to do your will.” He abolishes the first in order to establish the second. 10 And by that will we have been sanctified through the offering of the body of Jesus Christ once for all. 11 And every priest stands daily at his service, offering repeatedly the same sacrifices, which can never take away sins. 12 But when Christ had offered for all time a single sacrifice for sins, he sat down at the right hand of God, 13 waiting from that time until his enemies should be made a footstool for his feet.

Christ’s death was necessary because there can be no forgiveness without the shedding of blood and the sacrifice had to be a spotless lamb. How could anyone in the nation of Israel die for the sins of the nation? No one was sinless. No one could die for the sins of others when he bore his own sin. He would have to atone for his sin first and then the sins of the nation. The only one who is sinless is God himself, so God sent his Son to die for the sins of the nation (and, as we see in 1 John 2:2, the sins of the whole world). He had to be born of a woman so that he could represent man to be his sin bearer but he also had to be God because only God is sinless.
And in Ezekiel we see that the purpose of the cross was to vindicate the holiness of God. The world has seen what it takes to atone for sinning against God; what it takes to purify a people so that they could be called the people of God. A sinless perfect man and very God himself had to take our place and bear the wrath of God. That is the payment for sin, he doesn’t wink at it and say, “Poof it’s gone.” He expects it to be paid in full. Christ paid that price for those who are united to him but those who aren’t in Christ will pay for their own sin throughout eternity.

2 Corinthians 9:15 Thanks be to God for his inexpressible gift!