I'm surprised no one mentioned the use-of-force continuum. It may not completely apply in this situation but it does make sense here.
First and foremost, if I were with loved ones, I'd send them away to a safe place. In this situation I would have yelled for the guy to stop. I carry pepper spray because one can't always utilize deadly force. If he didn't stop pummeling the victim, I'd spray the guy to at least slow him down. After spraying, I'd get the hell out of there. At least that would break up the beating. Shooting the guy would be the last option.
"You haven't lived until you've died." - various