Albert Barnes Commentary Psalms 109:21

"But deal thou with me, O Jehovah the Lord, for thy name`s sake: Because thy lovingkindness is good, deliver thou me;" — Psalms 109:21 (ASV)

But do thou for me, O God the Lord, for thy name’s sake - That is, interpose for me; exert your power in my behalf. The phrase “for thy name’s sake” implies that the motive which prompted him was a desire that God might be honored. It was not primarily or mainly for his own happiness; it was that God might be glorified, that his character might be illustrated, that his plans might be accomplished. Compare the notes at Daniel 9:18-19.

Because thy mercy is good - That is, it is the characteristic of mercy to do good; to show kindness.

Deliver thou me - He prays that God would “manifest” himself as he really was, as a God of mercy.