DISCLAIMER: I don't own Fruits Basket.
Kyo couldn't understand what he did wrong. His whole life he asked himself what did he do and he always got the same answer. Nothing. He didn't do anything but his family hates him. He was born and everyone hated him for it; is it a sin to be born? He can remember all the cruel things said about him when he was younger. How it was his fault his mother died and how he was a monster, even now he still gets called a monster. Kyo became angry because he didn't understand their reason to hate him.
So Kyo tries to give them a reason. He lets his anger get the best of him all the time. He yells, and fights, and acts rude and disrespectful. He figures that he can blame their hate for him on his anger. It's less painful for him this way. He knows that no matter what they are always going to hate him so why not give them that reason.
It broke Kyo's heart seeing the way his family treated Tohru. From the moment she entered the Sohma's lives she was treated as if she were a princess. He didn't hate Tohru for it, no he hated his family. They treated Tohru, a stranger, as if she were a goddess and they treated him, their own family, as if he were worthless; as if he didn't deserve to breath. Tohru gave them no reason to love her but they did and she was always happy. Kyo gave them no reason to hate him but they did and he was miserable. So Kyo kept up his anger act even though all he really wanted to do was cry. He kept up his act because being hated for a reason was slightly less painful than being hated for no reason.
When ever I read Fruits Basket and see how bad everyone treats Kyo I get mad. It's not his fault that he was born as the cat. It just made me mad to see how kindly they would treat Tohru and be so mean to Kyo. I just couldn't understand why they all treated him differently so I made this.