A policeman's job is to enforce the law, not kill criminals. In this case that means arresting the bank robber so he can be brought to trial. If the bank robber comes out of the bank "guns blazing" then the police are certainly justified to shoot him as a matter of self-defense.

If, however, the bank robber steps out unarmed and is shot and killed, the family should have the right to bring a wrongful death suit (or something, IANAL) against the police officers in question.