In Season 2, Episode 4, “Will the Wise,” Billy observes Max and Lucas talking from the school parking lot. When Max gets in the car, Billy threatens Max, saying, “There are a certain type of people in this world you stay away from,” and that Lucas is “one of them.” At this point, Billy has never spoken to Lucas and doesn’t even know his name, yet his snap judgment is the basis for future bullying he directs at Lucas just for hanging out with his stepsister.

Source: https://www.distractify.com/p/why-did-billy-hate-lucas-stranger-things