Part five
Tommy appears to attack his therapist (the young woman) in Voorhees garb at the end, but it is never clear whether or not this is supposed to be an hallucination. We don't even know the outcome, though the novel of Jason Lives says that she brought him to his senses. 😄
The perpetrator with the hockeymask also had a papermache skull mask to look like Voorhees. The murderer was in fact Roy Burns, the paramedic whose son was the fat boy killed by Victor Fadden early in the film. 😈
Burns was angry! 😠
Tommy killed Burns by shoving him off the loft of the barn and onto some sharp objects, possibly a rake. The viewers get to see that he was not really inhumanly ugly, just disguised as Jason Voorhees (Tommy had disguised himself likewise in the Final Chapter...)