I'm just hate the fact that they basically had to ret-con the storyline of the first movie to make Jason the killer instead of his mother.

In part 1, Mrs. Voorhees motivation for killing is that fact that she watched her son drown in 1957. Then the majority of the movie takes place 21 years later.

Then is the second movie, Jason somehow survived his drowning and grew up completely alone in the woods (for 21 years!) and his motivation for killing is revenge for his mother. I know suspension of belief and all that considering Jason "dies" multiple times in the franchise. But that has always bugged me.