Albert Barnes Commentary Psalms 119:121

"I have done justice and righteousness: Leave me not to mine oppressors." — Psalms 119:121 (ASV)

I have done judgment and justice - This begins a new division of the psalm, indicated by the Hebrew letter Ayin (ע ‛) – a letter that cannot be well represented in the English alphabet, as there is, in fact, no letter in our language exactly corresponding with it. It would probably be best represented by what are called “breathings” in Greek. The meaning of the first part of this verse is, “I have led a righteous and upright life.” This is equivalent to saying that he had kept the law of God, or had made that the rule of his conduct.

Leave me not to mine oppressors - To the people who would do me wrong; who seek my hurt. He urged this on the ground that he had been obedient to the divine law and, therefore, could properly make this request or claim divine protection. A person has no merit of their own and no claim on God; but when a person is God’s true friend, it is not improper to expect that God will intervene on their behalf; nor is it improper to present this in the form of a prayer. Our loving God and serving Him, though done imperfectly, is, in fact, a reason why He should and will intervene on our behalf.