Matt Duffer says, "[Billy's] just a terrible person," while Ross Duffer adds, "[Billy] is racist, I think. It’s [reflective of] people that he’s viewing as different from him and an overall rage at the world. In this case, seeing his sister with someone like Lucas leads to that conversation."
Matt continues, "But, also, any way [Billy] can find a reason to attack Max, he’ll use it. We wanted to be honest about what an [interracial] relationship like that would do, and how certain characters would react, and how a character like Billy would really react to that."