You're all missing one key thing. Junpei was motivated by his obsession with the idea that Mitsuru was scum. Mitsuru as in not scum Mitsuru. As in Cop Mitsuru. He was trying to get Mitsu's scum buddies to come to her aid. He wasn't onto any actual scum! He even straight up said "Koromaru: don't think you are playing as mafia" right before you claimed.
