The original play was a brilliant tragicomedy (tragedy mixed with comedy). The movie follows the play rather closely, except with two major differences.
1. The movie tries to make the protagonist, Seymour Krelborne, look more sympathetic than in the play.
2. The movie has a standard Hollywood happy ending, while the play ended with Seymour getting his just desserts.
The play, while creepy, was also brilliant. I'm a HUGE fan of the play. It made me laugh, cry, and shake in my seat, all in just 80 minutes plus intermission. The movie does not do the play justice. It takes the play, which was basically Faust set in an old B-movie, and warps it to the point where the entire message has been diluted into nothing.
A more depressing ending was filmed and shown to test audiences. The test audiences cried during it, so it was removed. Well, OF COURSE! That's the charm of the play! Not only do you laugh, but you ALSO cry! That's why it's so brilliant!