I feel bad for him too. He abused Niannian because he thought Niannian was someone who destroyed his life. This doesn't justify his actions, that true. But lets not forget that he has like major anger issues which has been potrayed in almost all the episodes. When anger takes control over you, no actions are justifiable. And also, he thought that Niannian was someone like the real Shen Luoan. A lying rat who tried to claim that he was Gu Yansheng's savior and that he loved him when he was only after his fame and money. And I think we all know how we feel when we think about Luoan. We want to tear him limb by limb, dont we? And thats when we only know him from a few chapters. Think about how much you will want to do that if you see him every single day. And also, he was totally brainwashed by Luoan. He made Niannian look like the villian. I think it was just too much for Yansheng. He was forced to live with the person he hates the most (well, he came to love him later but whatever) and had to see him every day. Also, no one accepted his love for Luoan...seems like it is a really great trigger for his anger issues...so he ended up showing it physically toward Niannian. As I said, it doesnt justify his actions and it isn't fair.......but if you think about it, it is understandable....life really is a confusing thing, isnt it?