I assume I have a permit for the pistol. In the hypothetical am I an off duty law enforcement officer?.
Robber is likely to have people on watch and with get away vehicle- shooting robber may cause a fire fight and injure friendlies.
I agree with the comment about demeanor- if robber remains calm and your life or the life of the bank teller is not in danger you cannot legally shoot.
If you take action you become responsible for the consequences. Morally I would like to take out the bad dude, but law and morality haave very little to do with one another.
Get ready for the risk of escalation- if they do I agree with the centre of mass comment and be ready for multiple targets as anylook outs- who may not be readily apparent- become animated.