There's something deeper that was already wrong with him. Even a kid who sees a traumatizing scene like that normally wouldn't become so abusive to his own brother, and continue it even into adulthood. It's upsetting to see your parents doing that, but not enough to turn you into a sociopath. He wasn't even a little kid when he saw it, he was a teenager. He definitely needs mental health treatment and to have his license revoked.
Oh, and he's probably a pedophile too! If his brother is in high school or maybe college (I can't remember if they said which) at the same time he's completed medical school and is an established doctor at a hospital who has gotten to the position of a department head, that means there's a huge age gap between them, so when he started abusing his younger brother, his brother was probably around 6 years old. This man is one of the most disgusting people I've ever seen in a manga.
That fucking brother should die a painful death ! All this bc he saw a BDSM session between his parents when he was a kid !! It literally screwed with his mind ... tbh he should undergo psychiatric treatment, his medical credentials taken away, bc the bastard is a sadist of the worst kind !!