While the two were justified in their interest of seeing the scumbag get his just desserts, killing was not the right answer. I didn't agree with letting him walk, nor did I want butterfly dude to die, but he threw his life away at the end rather than taking a better turn. He had a sliver of a chance at redemption, no matter how little it was. Bandou deserved to die, but killing him would not have been the right answer. Lead senpai did the right thing in the end though, leaving that scumbag lawyer with a totally messed up face.
RIP nohomo bug bros.