Laserblast

Laserblast is a 1978 American science fiction film about an unhappy teenage loner who discovers an alien laser cannon and goes on a murderous rampage, seeking revenge against those who he feels have wronged him. The lowbudget film was directed by Michael Rae and produced by Charles Band, who is widely known for producing B movies. Starring Kim Milford, Cheryl Smith, and Gianni Russo, the film features notable cameo appearances by Keenan Wynn and Roddy McDowall, and marked the screen debut of actor Eddie Deezen.

A greenskinned man wanders through the desert with a laser cannon attached to his arm. A spaceship lands and two aliens emerge, one of whom shoots the man, which disintegrates his body. The aliens depart on their spaceship, leaving behind the laser cannon and a metallic pendant the man was wearing.Teenager Billy Duncan wakes up in his bed, seemingly disturbed, and learns his mother is leaving for vacation. He goes to visit his girlfriend Kathy, but her deranged grandfather Colonel Farley makes him leave before he can see her. As Billy drives around town, he is harassed by bullies Chuck Boran and Froggy, and by two police deputies who give him a speeding ticket. 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. Meanwhile, on the alien spacecraft, the two aliens converse with their leader who shows them footage of Billy using the cannon, prompting the aliens to turn their ship around to head back to Earth. Context implies that the two aliens, upon departing Earth, left the cannon and pendant behind under the presumption that no other human would be able to use them as the greenskinned man had, but they have now learned that they were in error. ........

Source: Wikipedia