A Twisted Individual
Saruhiko Fushimi is described at being twisted and in battle against members of Homra (especially Yata Misaki) it's not hard to see why. He becomes bloodthirsty and begins to say things that no sane person would say. That being said, Saruhiko is far from mad.
While Saruhiko grew up with abusive parents who failed to properly socialize with their son he didn't show any signs of the bloodlust or insanity, just antisocial tendencies. Even when he became friends with Yata Misaki he continued to be sullen and antisocial when dealing with individuals beyond his closest friend.
After joining Homra, Saruhiko was often in battle. He would fight back to back with Misaki and yet he never showed the sadistic tendencies that became his trademark in Scepter 4. So what changed?
Before leaving Homra, Saruhiko went through a time where he hallucinated his father's ghost. This made him violent but not bloodthirsty. While it was eventually discovered to be the work of the Green Clan it is possible (but unlikely) that he continues to be effected by it. He was also reaching adulthood around the time he joined Scepter 4. It is therefore also possible, but again, unlikely that Saruhiko developed the signs of a serious psychological disorder such as schizophrenia. Saruhiko doesn't show many of the classic symptoms of such disorders and those that appear only do so when he is in battle.
I would therefore surmise that Saruhiko's behavior is an attempt to drive Misaki away. Saruhiko grew up without the affection of parents. Misaki's entrance into his life changed that. Suddenly he had a friend, someone in this world who wanted to be near him, who liked him for who he was. Therefore when Homra entered the picture and suddenly he wasn't the center of Misaki's world he began to think the attention Yata gave him may have just been another trick.
He withdrew first from Yata and then from Homra. He wanted to prove that he could abandon Misaki and not be abandoned and his actions while fighting Homra is just a reflection of the mask he put up to protect himself from being hurt by his best friend.