I think there will be no explicit sexual scenes... so we probably will never know the roles... this is shounen ai after all.
Mo wasn't a badass, but a douche. He was clearly hated because he bullied Jian Yi and knocked Xixi.
Knowing a bit of his background (his family used to have a restaurant and he is truly desperate to have money), that's just a kid doing pranks and bad things because deep down he want to be noticed as a "cool kid who likes to bully weak people". But he was noticed by a far more dangerous kid.
As for his sin, I think he had already a fair share of punishment with He Tian (he was threaten, kicked, has his balls twisted, beaten and so on). Right now, I think He Tian should cut him some slack. That's really abusive right now.
I didn't want to rant about this (the last chapter makes me happy and angry at the same time and that's frustating) but for now I think He Tian is going too far and I might begin to hate him instead.
Just re-reading the chapters and man,
I remember when Mo used to be such a badass and everyone hates him.
Who could have guess he'll get tongue rape by He Tian, hahaha, I bet it's jiggling inside. It must be the punishment for his sin, you know, hitting Zheng Xixi in the side of his head and all ╮( ̄▽ ̄)╭