Charles H. Spurgeon on Psalms 109:6

Set thou a wicked man over him. The first thing that the Psalmist asks is, that his foe might be subjected to the evil of having a man placed over him like himself: —a man regardless of justice, truth, and right; a man who would respect character and propriety no more than he had himself done. It is, in fact, a prayer that he might be punished in the line of his offences. It cannot be wrong that a man should be treated as he treats others; and it cannot be in itself wrong to desire that a man should be treated according to his character and deserts, for this is the object of all law, and this is what all magistrates and legislators are endeavouring to secure. —Albert Barnes.