You did the right thing in the situation, but it is natural to feel helpless afterwards. Putting your life in danger to stop someone who is making off with money isn't the right thing to do. You don't want to risk harm to yourself and ANY member of law enforcement will tell you you did EXACTLY what you are supposed to do.
It sounds like the gravity of the situation hit you like a truck afterwards.
On the bright side, you will be getting a raise within the next few days
(Seriously).