If you really think about it, the stickers aren't that OP of a power, certainly not enough to have a cult blindly follow your every command. Plus their use is pretty limited
This plot makes no sense. The douche literally killed the teacher and some girl outside the circle of the sticker users. The MC would have to kill him first.
@Iskarioth I think Mario realizes he can't get rid of all the sticker users without him so his plan is likely to use him to get rid of the rest and then save him and Mori for last. While he is the biggest threat and getting rid of him sooner rather than later could be a better plan, it'll be harder to do so if he sees Mario as an enemy still and then Mario has only Mori left to help him with the rest so his chances of success are much higher with this route from that point of view.