Here yoichi is not bad so to speak, he can feel compassion, but when he loves someone. Platonic (Zen) or romantically (kudou) then his morality is devalued. And it makes him hungry , Zen knew that if his brother fell in love he would probably abandon him, so he did everything possible so that Yoichi would never fall in love with any women (Which was dumb, because he never asked Yoichi what his sexual orientation was, so he kept him away from women but not from men) Yoichi saw Kudou from afar , let's say he saw it on a mission or something, idk, And his brother then told him how annoying a certain "kudou" was and yoichi fell in love, Yoichi, innocently believing that his brother would support him told his twin about the new feeling he had, Zen did not want to lose his brother, And less that there was a sewer rat like that Kudou, so he locked Yoichi in his room, but he found a way to get out, And he refused to spend time with his brother like before, so Zen locked him in a vault because yoichi somehow escaped from his room or made a mess in the bathroom, (He also began to have psychotic breaks) yoichi, out of desperation of not seeing kudou begins to eat his own wrists, Kudou and Bruce invade Zen property, find Yoichi, Kudou assumes that those wounds were caused by Zen, Yoichi puts on his cutest eyes and happily allows himself to be robbed.