Harry's Player: DM, I'm going to do that thing that I talked to you about.
DM: Okay, you get into position to do that thing, and just as you're about to finish it you hear a voice behind you ask "What are you doing?" [Paladin's PC], you wake up in the middle of the night to the sound of a sword softly scraping its way out of its scabbard. Rolling over you see Harry, sword in hand and positioned to plunge it into Derek's throat. Before your sleep-muddled thoughts can organize themselves you hear your mouth ask "What are you doing?"
DM now steps back and lets the PCs RP their way through their interpersonal conflict. Harry cannot kill Derek without consequence any more, Derek isn't in a position to escalate the situation, and the another member of the party is included in the conflict.