The younger Sou cheated on the younger Rin while the younger Rin gets raped by the future ...

But Rin also grew up to be quite vengeful too...

idk, seems like adult Rin was more going with it because he remembered what Sou did to him when he was younger and was more hurt than angry. Sou came across as being very cruel to the younger Rin. Got a bad vibe from him. Their attitudes were different but yea, in the end they both did each other's younger versions which I'm still trying to wrap my head around lol