Billy is racist

I think he would have terrible to anyone associated with Max regardless, but that just added another huge prejudice to the mix.

Racism doesn't exist. It's an illusion when in reality the issue is a spiritual issue rooted in anger.

We don't really understand Billy's motives yet. He's never come right out and said what his issue is with Lucas, although referring to him more or less as the "wrong type" does give the impression of racism. He doesn't know a thing about Lucas. How else to interpret it?

What I gathered of the family dysfunction is: his father married Max's mother and because of that, they moved away and Billy had to leave behind his old life and his friends, for which he apparently blames Max. It isn't her fault of course. Billy is too scared to stand up to his old man though, so he takes it out on his stepsister instead.