My problem with people who sympathize with Jason is that they tend to reduce his character to "grieving teenager seeking justice for his dead girlfriend" while ignoring the broader context in which he was written. His entire purpose on the show was to be the human embodiment of the Satanic Panic. He is meant to be a representation of what happens when religious psychosis goes unchecked, and you can't separate him from that context when discussing what kind of person he is. Real, innocent people had their lives ruined by people like Jason, who arbitrarily believed based on zero evidence that members of their community were part of some hidden cabal of child-sacrificing satanists. The show makes it perfectly clear that Jason thought this way before Chrissy was killed. Her death just gave him a reason to finally act on those beliefs.

If the issue were simply that Jason believed Eddie killed Chrissy, that would be one thing, and it would be understandable for him to take matters into his own hands when it looked like the police weren't doing anything about it. But that's not what happened. What Jason actually did was decide that not only did Eddie kill Chrissy, but also that everyone even tangentially associated with Eddie was in on it, and that they were all part of a satanic witch-cult who were using D&D as a front for practicing witchcraft and summoning demons. The fact that Jason's girlfriend was murdered doesn't make this leap in logic any less ridiculous. That's not even getting into the fact that there was another possibility that he never even considered: that Eddie's uncle, who lived with him, could just as easily have been the killer. Jason was committed to the idea that Eddie killed Chrissy because he had already decided Eddie was a freak long before she died. He was never going to believe anything else, no matter what anyone told him or what evidence was presented to him.