@Ainz
It has to do with how you define pacifist.
The standard definition of pacifist is someone who won't fight, no matter what. Which means, yeah, when dealing with someone who will use violence as part of aggression, they're just a punching bag. A born victim.
The MCs definition of pacifist seems to be, "I won't start anything myself, but if you decide to start something... I'll finish it."
In other words, he won't be the aggressor. He won't deliberately provoke someone for the hellz. He feels no need to prove his manliness by being a "tough dude". He doesn't get his jollies by being a bully.
But... he will defend himself, and others, from unjustified actions on the part of others. If someone targets him, he will respond at whatever level seems called for, including dealing out death.
Given that, as far as I've read (Chapter 12) he doesn't have a way of suppressing someone's ability to utilize Divine Magic, he seems to be taking the stance that you only get one free shot, and then you're on the elimination list (provided you have Divine Magic; non-powered individuals can be restrained, so the possibility of talking the situation out is there). If he can't safely restrain you, then he has to permanently remove you as a threat.
It's possible I'm reading stuff into the MC that isn't actually there. But so far, it really does look to me that while he won't push others, he also won't be pushed. He won't sit by while unjustified actions are taken by others.
Whether or not he feels someone's actions are justified will depend upon his understanding of the situation, of course, So the possibility does exist for him reacting at the wrong level.