I have role-played thousands of scenarios with the help of computer games, and watched a large quantity of violent Hollywood movie scenes, so I can easily imagine both killing in self-defense and pre-emptively murdering people that may or may not actually deserve it.
The best I can say is that it would be morally unacceptable for anyone to murder you, specifically--a person who has publicly renounced killing. And because you desire to not kill in your own defense, it is therefore more acceptable for me to kill someone else in your defense.
But doing so would offend your own values, so do I have time to judge whether that is a principle you would die for?
It gets complicated.
Fortunately, the law only deals with what is legal, and not with what is right. Sometimes, breaking the law is the only upright thing to do. If the law says it is still murder to kill in self-defense, then you kill that assailant, and then you take whatever punishment the state dishes out for it--not because it is just, but because you shouldn't fight an adversary that you cannot defeat. Obviously, it is better to live under laws that match your personal principles, but we can't always do that.
Pacifism is a perfectly reasonable moral stance to take, but I do not think it could exist without non-pacifists willing to use force to defend those pacifists. And that is why I am not a pacifist myself. Obviously, it would be better all around if everyone could be a pacifist, but that experimental world quickly falls apart whenever anyone goes renegade.
