BILLY WAS A KID WHO GOT PUSHED AROUND... THEN HE FOUND THE POWER.
A happy go-lucky teenager stumbles upon an extraterrestrial weapon, which transforms him into a grotesque killer. He becomes the target of a pair of aliens who recently executed the weapon's previous owner.
I gotta watch it! Here are a couple links I'm gonna try:
there are also a couple MST3K versions up on youtube tho one seems shorter than the runtime of the full movie.
EDIT: Aw yeah! It's got a supporting role by Eddie Deezen (famed for playing "nerd" characters in Grease and WarGames, as well as for doing various voice acting.)
Figure 1: a scene from WarGames