Because that isn't the scenario. You have to actively kill him in order to survive.
First, I already did with the screaming-panic-attack-guy(though for obvious reasons you'd knife him instead of shooting). And really, you do consider such a scenario impossible? Do you just want me to lay out the scenario so you can(pointlessly) poke holes in it or do you HONESTLY BELIEVE that a situation where you need to kill an innocent to save the lifes of others is FLATOUT IMPOSSIBLE?
Secondly, the scenario
does't have to be 100% feasible and realistic. Again, that is NOT how thought experiments work.
Let me give a famous example; perhaps you are familiar with Philippa Foot's
Trolley problem.
If your answer to the premise is that the situation is unrealistic and it would never happen to you like that, or that there is a third option you'll take, or that there is always a chance the guys on the rails manage to leap aside in time... Then you are just making a fool of yourself. Really, go into any philosphy class and reject this and similar thought experiments because they are not a 'feasible scenario'. Depending on the other people's attitude they might not even try to hide their sneers and derisive laughter.
Same goes to Traab's last post. Rejecting the premise of a thought experiment is the most pointless and useless answer to it you can give.