I'm looking for a movie (Circa early 80s, maybe late 70s) about a teenager who finds a laser pistol and succumbs to "Power corrupts" eventually going on a killing spree. I remember that the gun required an amulet than eventually began to change the teen. I think the teen was one of those kids that was made fun of and thus used the gun for retribution. I also remember the film not being very good so I've not had a lot of luck with Google on this one.
It sounds like Laserblast (1978).
". . . Billy wanders into the desert and discovers the laser cannon and pendant. He starts playing with the cannon, pretending to shoot things, then realizes he can fire the weapon while wearing the pendant. . . ."
An edited version was shown on MST3K.