Friday the 13th Part VI: Jason Lives

Superhuman serial killer Jason Voorhees has been dead and buried for six years. However, Tommy Jarvis, who killed Jason in self-defense when he was 12 years old, is still haunted by his encounter, resulting in his having been institutionalized for an extended period of time.Intent on cremating Jason's body to rid himself of his demons, Tommy escapes the mental hospital with his friend, Hawes, and breaks into the cemetery containing Jason's grave. He and Hawes exhume Jason's casket, but before they can cremate the body, an infuriated Tommy begins stabbing it with a steel fence post. The post acts as a conductor for an ongoing lightning storm, and Jason is resurrected from the dead. He kills Hawes with a punch through the heart and throws his body into the exhumed coffin, prompting Tommy to flee the cemetery.Tommy returns to the town of Crystal Lake, the site of Jason's killings, which has now been renamed Forest Green to distance itself from negative publicity. Tommy attempts to warn the town arrogant sheriff, Mike Garris, of Jason's return, but Garris, aware of Tommy's institutionalization, writes him off as disturbed and has him locked in a holding cell.Meanwhile, Jason begins a trek back to the lake that was the site of his drowning as a child. En route, he encounters Lizabeth and Darren, a pair of summer camp supervisors, who are themselves headed to the lake to supervise the re-opening of the summer camp. Jason attacks and kills them, leaving their bodies in the woods.The next morning, Sheriff Garris's daughter, Megan, who is slated to be one of the camp counselors, arrives with her fellow counselors Cort, Sissy, and Paula to report Lizabeth and Darren missing. Tommy cites their disappearance as evidence of Jason's return, but is met with hostility from everyone but Megan, who takes a liking to him. Sheriff Garris sends the counselors off to the campsite and then escorts Tommy out of town; en route, Tommy flees to the cemetery to try and show Garris the open grave, only to discover that the groundskeeper, fearful of being implicated for digging up the grave due to his alcoholism, has covered the grave (and, consequently, Hawes's body) with dirt. Garris handcuffs Tommy and takes him to the city limits, warning him not to return.Meanwhile, a quintent of business people playing paintball in the woods are set upon by Jason, who kills them and steals their supplies. That night, Jason continues making his way back to Crystal Lake, in the process killing the grave digger and a nearby couple having a picnic. Cort meets up with a local girl, Nikki, and leaves the camp to have sex with her in the woods; they end up in Jason's path and are both killed by him. Sheriff Garris ultimately finds the bodies, and believes that Tommy has killed them, living out a delusion of Jason's return.Tommy, meanwhile, has contacted Megan, having figured out a way to defeat Jason after having read books on monsters and folklore: He can be incapacitated by being trapped beneath the surface of the lake where he drowned. Megan attempts to bring Tommy back to the camp, but they are intercepted by one of Garris's roadblocks. Tommy is arrested and Megan is escorted back to the police station to await her father's return from the field. The police's attention on Tommy permits Jason to slip into the summer camp, where he kills Paula and Sissy, but refrains from harming any of the children.Megan and Tommy escape the police station and make it to the lake, where the pursuing police are forced to acknowledge Jason's return when he attacks them. Garris and his deputies briefly incapacitate Jason by shooting him with high caliber weapons, but Jason ultimately recovers and kills them all. He then attempts to kill Megan, but is distracted by Tommy, who beckons to him from the lake. Seemingly remembering Tommy, Jason abandons Megan and wades out to the lake, where Tommy ambushes him with a chain attached to a large boulder. A fight ensues, during which both Tommy and Jason are knocked into the water; as Tommy attempts to swim to the surface, Jason pulls him underwater and he loses consciousness. Megan swims out to save Tommy and is likewise attacked by Jason, but finally incapacitates him by driving a motor boat propeller into his head.Back on land, Megan revives Tommy with CPR and the children celebrate. At the bottom of the lake, Jason floats, attached to the boulder, still alive but powerless to escape.

Question: Why was Tommy institutionalized as a child?
Answer: Killed Jason