Albert Barnes Commentary Psalms 54:5

"He will requite the evil unto mine enemies: Destroy thou them in thy truth." — Psalms 54:5 (ASV)

He shall reward evil to my enemies - Margin, “those that observe me.” The original word here means literally “to twist, to twist together;” then, to press together; then, to “oppress,” or to treat as an enemy. The reference here is to those who pressed upon him as enemies, or who endeavored to crush him. The idea is that God would recompense them for this conduct, or that he would deal with them as they deserved.

Cut them off in your truth - In your faithfulness; in your regard for what is right. This is simply a prayer, or an expression of strong confidence, that God would deal with them as they deserved, or that he would not allow such conduct to pass without a proper expression of his sense of the wrong. There is no evidence that David in this prayer was prompted by private or vindictive feeling.