Brad is the hunky quarterback who's been conducting illicit sexual escapes with Patrick on the eighteenth green of the golf course. Brad's ashamed of being gay and is trying to hide it from everyone—maybe even himself.
It doesn't help that Brad's dad is a total lunatic, beating him and treating him like scum. But that definitely doesn't excuse Brad's actions, like calling Patrick a "faggot" (4.3.9) in front of the whole cafeteria and, with his buddies, beating him up five against one. He does feel bad about it, telling Charlie in detention, "Thanks for stopping them" (4.3.27). Yeah, that's nice, dude, but you should have stopped them yourself.