I think you're too generalizing about mental illness. Types of mental illnesses are wildly different from eachother and are obviously not going to result in the same state of mind. Comparing Dogisaga to Chris' comics seems kind of useless to me for that reason. Sure they are both 'uncanny', but in very different ways. Dogisaga clearly has no problem grasping facial expression, other people's emotions and nuanced jokes. In fact, one of the most notable characteristics of her comics is that she pokes fun at her own BPD and dysfunctions.
Even within mental illness BPD and autism are somewhat opposite ends of a spectrum. What sets Chris' comic apart, amongst other things, is the absolute lack of self awareness and nuance it shows. Chris has a very strong sense of justice. That's why everything Chris does in his comics is always the good thing to do, what he would call the 'true and honest' thing. Compare this to Dogisaga, in which justice or a good/bad dichotomy in a classical sense are of very little importance.>Chris will have to kill a person getting in the way of his love quest(per.log), but this person is a demon man who is doing it just because he's an asshole and he just laughs at Chris the entire fight (un.emp)
To Chris, his love quest is without a doubt the most important and good cause. That's why there's no way that someone opposing that cause could be anything other than evil. Obviously, when your opponent is an evil demon man the good thing to do is to stop him, especially if that demon man is getting in the way of your unequivocably righteous cause.
There's not really any odd logic at work here, because the entire story exists to create scenarios in which Chris' actions are without a doubt good.