In this day and age not much surprises me anymore, so this could happen. Plenty of sick people out there.
Tvc would probably be best at answering this one, but I will take a stab at it. I don't think you could be held any more responsible than if someone grabbed a cop's gun and shot several people. Plus, I'm not so sure that the sight of a gun would necessarily inspire someone to grab it and use it. I feel that if they want to shoot into a crowd, it would most likely be planned and they would have their gun of choice. But, like I said earlier, nothing surprises me anymore.