If you look at chapter 14, theres a bathroom scene between Brett and Alex, and from the sound of it it's not the first time they've talked about the issue. Alex is a beta posing/hanging around alphas at the behest of his rich and influential parents, and Brett is telling Alex that he should do the things he wants to do because he chose to, not because someone else wants him to live a certain way. Alex is hanging around shitty alphas because he wants the influence and prestige associated with the group, despite the consequence of being mistreated for it. Alex stubbornly shrugs off Brett and tells him to mind his own business.
Theres a lot more to intricacies of the story, and while Brett could have done more than what he did you should also remember than Nathan's family are politicians. The repercussions of possibly overstepping would not only affect Brett but his family.
watching that prick Nathan get shot in the head was lowkey satisfying!!!!!!!ヾ(❀╹◡╹)ﾉ~ but also, it's actually disgusting how Brett knew all about the bullying and harassment but chose not to give a fuck. Bystanders are just as guilty so I'm not surprised the guy took a shot at him too!!