"Too bad for you that I don't really care what the crew of this ship thinks, then, huh?" Jack had crossed a few too many lines for Mordecai to even want to pretend to tolerate him. Sure, people would insist that there wasn't any proof of his crimes here, that it was Jack's word against his and Lilith's, and that that wasn't enough to convict anyone. He understood that. And so that's why he fully intended to kill Jack regardless of what anyone else said or did. A monster like him didn't deserve a second chance.