Umm, no. You obviously didn't read the story very well. Sarasa has reasons for being the way he is. Psychopaths are the way they are because they're born that way and they don't care about anyone else. Sarasa obviously cares about Kazunari and his cousin and vice versa. You want to know who true psychopaths are? Try Sou and You from Kimochi No Yukue. Yes, they can have real relationships, but they also need to be taught which is EXACTLY what Sarasa's cousin was doing for him. Kthxbainow.
Don't wanna pop your bubble but @CandyVonBitter is right. He's a psychopath. You don't need to be born a psychopath to be one, things such as traumas can influence, same with sociopaths and quite a lot of mental disorders. He is a psychopath, a light one at least, and he's a demented sadist who got messed in the brains, thus being the way he is. Period.
Where is all this talk about trauma coming from? Nowhere I saw does it say he experienced a trauma that made him how he was. Yes, he lost his parents, but at least once it's mentioned how they knew what he was, so clearly he was so before they died.
And not to be contrary, but I'm pretty sure you are in fact born a psycopath. Psychopathy is not a mental disorder or a disease, there is no cure, and you cannot teach a psychopath empathy. A lot of psychopathic traits can be symptoms of trauma or mental disorder, but that's different from a true psycopath.
I figure there is a parallel to true sadism, not to say it's the same, but Sarasa is clearly defined by the author as gaining pleasure, not through physical stimuli, but mental (and, I'm extrapolating, maybe emotional). And being a sadist he gains pleasure from other's pain.
If you are a sadist and live a normal life without hurting others, then you're either not a sadist or seriously repressed. And quite probable miserable. Conforming to "normal" and not getting your needs met is not healthy. A lot of mental disorders start that way.
Sarasa is a psychopath. Even the people who are really sadist can have a normal relantionship and avoid hurting people. I like the relantionship between cousing and Kazunari was fine, but the boy is a psychopath and it really bother me that they treated as just sadism. There is a fucking difference.