I'd always assumed that Psalm 109 was an imprecatory psalm. You'll recall that imprecatory psalms are psalms that call down curses upon enemies and adversaries. The most famous one being Psalm 137, with its chilling line, "Happy is the one who seizes your infants and dashes them against the rocks."
This is how the NRSV interprets it, too, to the point of starting verse 6 with the interpolation, “*They say*, ‘Appoint a wicked man…’” etc etc