Despite its title, No Longer Human is really about the pain of being human (duh). It's about depression. Alienation. Life spiralling out of control.
Okay, so Yozo, the protagonist, is afraid of humans. He doesn't understand why exactly people act like they do, but in order to fit in, he tries acting as human as possible. This poses an extra problem because now, on top of his fear of humans, he's also afraid lest people find out that he's a "fake," that everything he does is nothing but false affectation.
Now, I think that way of seeing things is interesting because isn't it normal for people to assume different "masks" when they interact with different individuals? It's to be expected that people act differently with, say, their boss than they do around their friends; it doesn't mean that they're being insencere.
Or perhaps that's precisely the point? Just because something is perceived as "normal," it doesn't mean that it comes easily and naturally for everyone. When you're incapable of doing what everybody else is doing, like Yozo, you might end up concluding that there's something wrong with you, that it's your own lack of humanity that is to blame. Yozo thinks like that, and maybe it just proves just how kind he is. (And why those women fall in love with him, even though all he seems to do is leeching them for money, sex and affection.) I imagine that people who are truly inhumane--psychopaths, let us say--wouldn't suspect that there's something wrong with them.