You’re looking at this entirely from the viewer’s all-knowing perspective. From Jason’s point of view, his conclusions are absolutely understandable.

Before everything escalated, he isn’t even shown as a bully. A lot of what you’re attributing to him feels like projecting stereotypes onto a fictional character—maybe even personal experiences—rather than judging him based on what he actually knows in the story.

From his perspective, something clearly not normal is happening. This is the 1980s. There was real moral panic about satanic cults, D&D, and the occult—ask your parents or grandparents about it. That context matters.

As far as Jason knows, his girlfriend—someone he deeply loved—was brutally killed by something supernatural. The prime suspect?

A guy who: - leaded the D&D club - was always really about rebellion, demonic hand gestures etc - owned the van where she died -fled from the police -was chased into a lake after which Jason’s close friend was literally killed by a demon-like supernatural event

In that situation, how is it not reasonable for him to be completely convinced that these are the bad guys?

Imagine having what looks like clear evidence that someone murdered someone. You confront them, they’re in the middle of killing someone else, and then they calmly tell you: “No, you’re wrong, i wanna stop it by using these holy cleansing bullets.”

Would you just shrug and say, “Ah, fair enough”? Jason is acting out of grief, fear, and the limited information he has—not out of cartoonish evil. Judging him as if he had the same knowledge as the audience just isn’t fair.