At what point can you determine whether someone means to kill you and whether he has a lethal weapon or not? Depending on how much bigger he is he may not need a weapon. If someone is beating on me or coming aggressively at me, I'll use deadly force and suffer the consequences. I'm not going to wait to see what his intentions are. Hopefully he'll back off when I show a weapon. If all he is doing is trying to take my stuff he can have it. In the OP's case, let him go. I would knock him off the bike if I could. He probably couldn't pick it up or even want to hang around to try.