Seems like this guy and his sister liked armed robbery, so I'm not personally too upset over his getting shot. However, if the guy was leaving the scene the most I'd have done is follow him until I could get the cops there. If he wasn't pointing his gun at me or someone else right at that instant I wouldn't have risked the possible jail time or legal hassles that could crop up from this.
I don't have a caped crusader complex and I'm way too fond of my stuff to risk it.
Sig 1911 Traditional reverse two tone, Sig p226 .40, Sig 556 Swat patrol, Baby Eagle/jerico steel .45