Well after a bit of thinking I would shot him. I doubt he would live even if I didn't shot him, so I'd rather at least have one person live (if the mastermind doesn't kill me anyway). Assuming a I'm let free, I'd try to find out all I can about the guy. Remember him and celebrate his life as a apology for killing him. Then I'd try to figure out who the mastermind is and bring him down. Make sure that no one else is hurt in such a way every again by him.