At first it looks like the writer of Psalm 35 is eager to take revenge, but look more closely. He is actually asking God to be his defender. He wants shame, dishonor, disappointment, and difficulty to come to those who plot against him, but these are not things he instigates.
“Let them be put to shame and dishonor who seek after my life. Let them be turned back and disappointed who devise evil against me,” and “Let their way be dark and slippery, with the angel of the Lord pursuing them” (Psalm 35:4-6 ESV).
In the middle of the chapter the psalmist writes of his sympathy for his enemies when they were in need—and their malicious actions.
“They repay me evil for good; my soul is bereft. But I, when they were sick—I wore sackcloth… I prayed with my head bowed on my chest. I went about as though I grieved for my friend or my brother. As one who laments his mother, I bowed down in mourning. But at my stumbling they rejoiced and gathered; they gathered together against me. Wretches whom I did not know tore at me without ceasing…” (verses 12-15).
Once again the psalmist brings it all back to trusting, serving, and praising God. We can trust him to deal with us his way, because he delights in the welfare of those who serve him.

“Let those who delight in my righteousness shout for joy and be glad and say evermore, ‘Great is the Lord, who delights in the welfare of his servant!’ Then my tongue shall tell of your righteousness and of your praise all the day long” (verses 27-28).
By Kathy Sheldon Davis