He is just a psychopath. No need for this pseudo justification. Psychopathy doesn't manifest in a singular way so its stupid to try and explain it in other ways just to make him look even more edgy and special.
Unfortunately, 'special' is exactly what the author is trying to go for. Psychopaths are real and not
that uncommon (and also misrepresented in fiction, but that's another issue entirely), so they're
lame. But MC-kun totally isn't lame, he's cool and deep and shit, so we gotta come up with some special explanation.
It's kinda like with Juujika where the story pretends the main villain is super cool for killing people for absolutely no reason, because no one's ever done that or something. In their attempts to make uniquely evil characters these authors just betray their own lack of skill.
And as a sidenote, the idea that MC-kun is severely lacking as a human being, but somehow almost perfect as a living being is pretty fucking funny. Yes, Demon King, the guy who, as you just explained, is quite literally incapable of cooperating with others and whose moral character (something that is not exclusive to humans) is completely inexistent is close to perfect. Sure.