Originally Posted by
Ryllharu
He chose Katsura at the end because all the other girls abandoned him after everyone found out he got Sekai pregnant. He didn't get back with Sekai after that because he refused to accept responsibility for Sekai's child. "Can't you just make it go away?" he said. 'Go Away,' like it's a door-to-door sales person. He's just a horny, irresponsibly little pissant of a human being, who thought he was hot shit because Sekai let him touch her. Katsura, having been driven insane earlier, knew none of this, and still wanted him back. She was the only person left he thought he could have sex with.
Makoto drove her insane, the other girls barely knew what was going on. After Sekai gave him what he wanted from Katsura in the first place, a one-sided relationship where he could still do whatever he felt like without caring about the girl. He wanted a fuck buddy. But because it was secret, he didn't have to go around with Sekai publicly. He never once defended Katsura when Otome and friends began to harass her. Never once admitted he was still seeing her, which to Katsura's knowledge, he was. He told Sekai he did so she would let him have sex with her, but didn't want to accept the responsibility of having to tell Katsura they were over. Then he pushed her over the brink when she found out what he did with Otome, she was being very badly abused by Otome's friends, and he pushed her away with Setsuna.
No one has ever been more deserving to die than Makoto. The tragedy of this series is that Sekai was killed afterwards. Katsura's insanity is a product of Makoto's actions.