Charles Ellicott Commentary Psalms 109:7

"When he is judged, let him come forth guilty; And let his prayer be turned into sin." — Psalms 109:7 (ASV)

When he is judged. —Literally, in his being judged. The meaning is, “may he go out of court a condemned man.”

Let his prayer become sin. —If this clause stood by itself, the most natural way would be to give “prayer” and “sin” their usual sense, and see in it the horrible hope that the man’s prayer to God for mercy would be reckoned as “sin.” That the performance of religious rites by a wicked man would have such a result was, it is true, a thought familiar to Hebrew thought. (Proverbs 21:27.)

But the judgment just spoken of is that of an earthly tribunal. Hence we must interpret it here as, let his prayer be an offence; that is, instead of procuring him a mitigation of his sentence, let it rather provoke the unscrupulous judge to make it heavier. For sin in this sense of offence, see Ecclesiastes 10:4, and compare 1 Kings 1:21.