Tsundere? I think he's more of an douche bag. He mentally and physically abused his brother for years, supported his parents in the abuse, stole his fiancée and rubbed it in his face. That's not how Tsundere behave, that's how genuine a-holes behave. And what amazes me the most is that MC wants to rebuild a relationship with him. If it wasn't for MC's strength, this would have ended with his brother just destroying him mentally and physically and taking everything from him. However, it's a Japanese manga and it's normal for them.

They say that kllng the Japanese manga/anime/LN protagonist's family, stealing his girlfriend, and then bullying him is the best way to become his friend lol.

Is he top or bottom?

a tsundere that tries to kill his brother after cucking him... sure

Unfortunately this is a standard manga/anime/LN cliche. I don't know what its roots are, but apparently in Japan it's considered normal for the MC to respond to bullying and attempted murder with care and love. To me, though, such injustice seems unilaterally stupid. Confucius once said: "If you return evil with good, what will you return good with? Therefore, return good with good and evil with justice" Because of this stupid cliche, I nowadays rarely read works by Japanese authors, preferring Korean and Chinese ones.