Where does Simon's responsibility end?

Yes, Simon is a bully, his actions in high school are abhorrent, and he doesn't seem to have changed as he aged. However, what I keep reading implied in the posts here is that Simon is responsible for what happened to Gordo. Here is my question: how is it that Simon is responsible for the actions Gordo's asshat father? Yes, yes, the law of unintended consequences and all that, but Gordo's father was a grown man who should be held responsible for his own reactions and actions. Simon didn't "cause" the father's actions, the father made his choices on his own. If it hadn't been for this particular rumor, what else might have set off the psychopathic father and triggered this level of violence against his son? It seems he was pretty damn willing to believe, and act on, the allegations with zero corroboration.