The more we read, Brett become more contradictory. He sometimes acting like he want something more with Hayden but also don’t want more than what they are now. So is Hard to tell he genuine have any feeling for Hayden or just playing around.
We can only guessing from information we do know about him. Brett came from rich family and his social skill is super good. He always have that fake friendly smile to anyone who interact with him. The relationship he have with other people pretty fake and distance
The three people he seem genuine close with is Martin, his best friend who “hating” him, his Brother who also feel uncomfortable with him, and you know, Hayden. So i have theory, maybe Brett love violent hardcore sex because it feel real to him? It’s not the faking like how people around him acting, but it raw and real. That why is exciting him and he addicting to it.
And that why the way Hayden behave gentlely and loving is weird him out. That not what he sign up with Hayden.