G-Senjou no Maou (English)

I think one person explained it pretty well, but there's another perspective to it. Maou basically disappears for the Tsubaki and Mizuha routes. That doesn't mean he wasn't their, and that Kyohei wasn't the true Maou.

In the case of Mizuha's route things might simply not have gone to plan, so he decided to slip into the shadows for a bit, go about things different. When you ultimately think about it, considering his ultimate goal he didn't necessarily have to do it in that city. It might have been a component of his ultimate plan, but plans can change.

In Kanon's route we see Maou cryptically stating "love" as his reason for ending his business in the underworld; he never said for who. Considering the objective of his actions, it's pretty clear to tell that Kyohei deeply loves his family, Kyosuke included. The only reason he turned on Kyosuke in the first place was because of what had happened to their mother. Although I admit this comes down to an unproven theory, it's quite possible he simply adjusted his plans to "circumvent" Kyosuke.

At least that's my alternative thoughts on the matter. Like it or don't, we're all entitled to our opinions.