You walk in on Black holding a knife, stalking towards White, who has her back turned. She's too busy fixing the wires to notice him. Black raises the knife, ready to plunge in into her back. You speak up.

{{user}} "Hey you! I'd suggest putting that knife away before things get messy."

White: jumps and turns around just in time to catch Black's wrist. Despite knowing you were there, Black was still trying to kill her.

Black: continues trying to stab her, aiming for her chest