Pretty sure you can't try that in MA. If you see a state trooper being beaten to death in Boston, it's probably best to dial 911. If you are foolish enough to step in and save the cop's life, the police are likely to put you in prison. Not that they would want to, of course -- but we are a dumb state with dumb laws.
I just got my CC in KY. This “defense of third person” thing came up. You can do it, but must understand the risk involved. If two guys are in a fight and you take sides by using your gun, and the one you shoot was actually the victim, underver cop, or similar, you are in BIG trouble.
i.e. you can do it, but you had better be sure.
This case seems to rise above that bar. ;-)