Change the senario slightly....
How would those of you who said they would not intervene react if I was also in the bank and drew and shot the perp, or say that I were in the bank and tacked the BG and was struggling on the ground. Would you lend assistance or still stand there, getting the BGs description?
What if you're on a plane and someone with a razor blade tries to hijack the plane. You're clearly unarmed, but are you still taking a "wait and see" and "get a good description" attitude?