Ah, that family tree cleared things up. I thought before he was after Qing Yu because they...

I thought the same thing :P

I know I was so worried that it was gonna be incest, but thank yaoi it's not (￣∇￣")

But then y is he after him again

Miss placed anger? The uke’s mum had an affair with the seme’s dad so the seme’s mum went nuts and killed herself but not before telling her young impressionable son that it was “they’re (dad/uke’s mum) fault” and they need to pay.
Uke’s mum is dead now (have a theory bout that) so the only one around to punish in the seme’s mind is the uke.

Right off the bat in chapter 1 it shows his father leaving and his mother says "remember it was your father and that low life slut (Qing Yu's mother) that killed your mom". Then she says "you have to avenge mom". So since the other woman is already dead, he's going to torment her son.

Oh ,, I see but to me he is helping the son instead of hurting him and even if it's to trust him,, it is not the poor uke fault that was his mother not him and since the mother died isn't it enough

Yeah he's helping to tutor him right now but that's only to gain his trust, that way it'll hurt even more when he pulls whatever crazy idea he has planned. Also you're right, it's not Qing Yu's fault which is why this whole thing is dumb.