God is faithful, the psalmist knows. Therefore he approaches YHWH on behalf of himself and against his enemies who are full of slander and lies. The psalmist is convinced he is right and they are wrong; he’s convinced God is on his side.
It is striking that the psalmist enters into God’s presence “morning by morning” because God is faithful — “because of your great faithfulness.” And here we are to think of God as YHWH (covenant commitment and obligation to be with Israel and to be for Israel), as King, as the one who is absolutely for honesty and integrity and implacably opposed to falsehood. So the psalmist lets out his plea:
“Lead me in your righteousness” (v. 8). That is, guide me through their falsehoods. The psalmist relies on God to protect and guide and to preserve. From whom? Those who perpetrate falsehoods, and the language of Ps 5:9 is graphic — appropriate to the occasion because God is absolutely other, God is absolutely truthful.
So the psalmist has two requests, once again casting a shadow over our squeamishness, and they are these: punish them because their falsehoods defy who you are (v. 10), and that God will bless them and make them become noisy in their joy (vv. 11-12). 
It is not news to see again how graphic and vindictive the psalmist can be; but we are to learn that God is just and that desiring justice is not a bad thing. The question we must ask ourselves is this: is our desire for justice vindictive, mean and violent or is it God-honoring and God-shaped? Do we want God’s glory and God’s faithfulness enough to stand against injustice or do we want justice so we can find ourselves on the top and in the right? Perhaps it is a cop out to say Jesus prayed for his enemies to be forgiven and find here a snooty superiority to the psalmist. Perhaps the forgiveness for which Jesus prayed entailed justice.

Our guide in the Psalms is John Goldingay (Psalms, Vol. 1: Psalms 1-41
).

For the music director, to be accompanied by wind instruments; a psalm of David.

5:1 Listen to what I say, Lord!

Carefully consider my complaint!

5:2 Pay attention to my cry for help,

my king and my God,

for I am praying to you!

5:3 Lord, in the morning you will hear me;

in the morning I will present my case to you and then wait expectantly for an answer.

5:4 Certainly you are not a God who approves of evil;

evil people cannot dwell with you.

5:5 Arrogant people cannot stand in your presence;

you hate all who behave wickedly.

5:6 You destroy liars;

the Lord despises violent and deceitful people.

5:7 But as for me, because of your great faithfulness I will enter your house;

I will bow down toward your holy temple as I worship you.

5:8 Lord, lead me in your righteousness

because of those who wait to ambush me,

remove the obstacles in the way in which you are guiding me!

5:9 For they do not speak the truth;

their stomachs are like the place of destruction,

their throats like an open grave,

their tongues like a steep slope leading into it.

5:10 Condemn them, O God!

May their own schemes be their downfall!

Drive them away because of their many acts of insurrection,

for they have rebelled against you.

5:11 But may all who take shelter in you be happy!

May they continually shout for joy!

Shelter them so that those who are loyal to you may rejoice!

5:12 Certainly you reward the godly, Lord.

Like a shield you protect them in your good favor.

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