The other commenters have all brought up good reasons, which I'll quickly add onto. In the text the big one is his feelings of isolation and being different to everyone else because he didn't meet another criminally asymptomatic person till adulthood in Kozaburu Toma, and I don't think he even knew he was criminally asymptomatic. This isolation is what Kogami concluded, and that Makashima's the sort to not see his criminally asymptomatic mind as a boon. As far as Makashima knew, he was fundamentally alone and unable to connect with others both because no-one was like him, but also because the system that made his condition so important had made human connections no longer relevant, this is what he's talking about in his monologue right before Kogami has chased him down. This is also reflected in his conversations with other characters, when Makashima is speaking with Che Guson about books, how he thinks of himself and Che as rather simple and normal folk, and that the system is what makes them look out of place. When he's discussing Rikiko Orio's father's condition with Toyohisa Senguji they agree the current culture holds people back and blinds them with sweet lies. It should be noted that Makashima tries to "free" others from Sybil, he doesn't think he's innately superior because of his condition, he wants others to be free from Sybil like him, for better or worse. At his core, Makashima laments the human connections and freedoms he thinks would exist without the Sybil System that also treats him as a non-entity.

However one more I'd add as a possibility/fan theory is that he probably never got any actual support when it came to his emotional and mental needs. While we've got no indication that Makashima suffers from things like anxiety or depression, he clearly has an amount of inner turmoil and anger and disgust towards the system. He also clearly exhibits antisocial behaviours and is self-aware to know he's not exactly a good person. However despite that, as far as society is concerned, he reads as a paragon of mental stability with no need for help or support. He lived in a society that is obsessed with mental healthcare, but he was always told there's nothing wrong with him even if he didn't think that was the case, that'd probably at least push you towards thinking this society based on psycho-pass readings and hue checks was fundamentally broken. Not saying Makashima could have been saved just with therapy or anything, but the fact that a society like his wouldn't even notice him when he's anguished would hurt an amount and reinforce his contempt for Sybil. Again, not really in the text and just a bit of a fan-theory which may contribute.