Utilitarianism vs deontology, ahoy!
Anyway, in reality, that sort of situation rarely exists. Generally they fall under "self defense" which most people wouldn't call murder anyway. So outside of fantasy situations and self defense, killing someone is pretty much never justified. You're never going to come across someone about to pull the End The World lever while you have a shotgun in hand.
It's extremely dangerous to use that sort of logic to justify political killings, too. Unless you think the Reign of Terror was an excellent idea, killing people for having different political opinions or being of a different social class is unambiguously
bad.
This dude is nuts. Just as nuts as a person who voluntarily shoots up any place, for any reason. There is no justifying murder.
You seem to be creating a false dichotomy - either he's nuts, or the murders are justified. It's possible to be responsible for your actions (IE sane) and still do something bad. Just writing off every criminal as "nuts" isn't productive or accurate.
Bit of a miscommunication here that's my fault; I use "nuts" in this situation to mean either actually insane, or just using horrible logic. Someone who does something stupid due to being a victim of their own logical fallacies would be "nuts" to me, not just someone who's mentally ill.
So the dichotomy here (that isn't false) is either A) killing someone is justified (self defense, etc), or B) they're insane, stupid, and/or ignorant.