>> If a civilian had shot him halfway through his massacre, would you
>> really have called her or him a "vigilante?"
>
> Yes, because that's the meaning of the term.
I think you need a much better dictionary[*]. The term you're looking for here is "self defense" ... the typical standards for the use of deadly force to stop this guy's attack were clearly present.
/jordan
[*] while you're at it, look up 'criminal'