You may remember this released as Rosemary's Killer, in the UK, in the early 80s. If it was'nt for the graphic violence, this movie could pass itself of as an episode of Murder, She Wrote. The premise is: GI, receives Dear John letter from his sweetheart, whilst he's fighting in World War II, overseas. Upon return, he discovers she's romantically involved with another guy; this tips him over the edge into being an unhinged maniac. At a Graduation Dance he disposes of them with a pitchfork, dressed in military fatigues and wearing a gas mask.
The film then transports itself to 1980, whereupon the same guy (whose identity is'nt revealed til the end) goes on another killing spree around the time of another Graduation Dance. I'm not going to give any spoilers, but I enjoyed this film, as it's a good mystery movie, however i have the un-cut version, and it's pretty nasty in places.