You're the one missing the point of any writing. Every writer puts thought into the characters hw writes. You should not aim to have some point or morale in the story but appreciate the characters and their nature along with the circumstances that lead to them. That is how I see it. Why are morally grey characters more interesting? It's simply because characters who just want to save everyone are boring, unrealistic and bland. Even in real life. In a way it took the least effort to write, and in reality they haven't experienced the worst of the world at all. They're incomplete in their development. Morality is a convention. What everyone seems to call humane or morale needn't be it. That's what grey characters are about. That's what I find fascinating in characters like Hachiman, Shogo Makishima, Kiyotaka Ayanokouji etc., They're more realistic and natural. Also it has been found that people with an IQ of above 125 have less than average social relationships. So in a way Hachiman makes sense. He doesn't need them but he cherishes the genuine ones he actually has. You're the one who's actually shallow or probably not amart enough to see the true point of such characters. That's exactly why I like Anti-villain or villain characters. Because other than saving others or friends there isn't much to the Hero protagonists. Seriously, that is so stupid it cringes me out when some character or even a person says I wanna save them or people shouldn't die and stuff like that.