@Wawawelon
Shimoda (the bullied guy) seems to just be a normal dude with some social issues. When he was in high school, he got into the "targets" friend group, but at some point, they "suddenly" changed and started to bully him.
At the same time, he poured his grievances to a third party - who is the insane guy who is now enacting "revenge"
The insane guy doesn't really need a good reason - he just saw himself and Shimoda as friends, so when Shimoda got hurt he made an obsessive plan to hurt people who hurt Shimoda.
I'm not completely sure why Rintarou is working with the insane guy, but it'll probably be some kind of mind break/Stockholm syndrome theme.
edit: Firstborn's explanation of Shimoda's relationship with the insane guy makes more sense.