I think the writer of the original objected to the idea of Jason being made into the villain in sequels, on account of the fact he always viewed him as the victim, and thus Pamela being a twisted "hero."
I don't know how he intended for the scene of "Jason" jumping out of the lake at the end of the original, but maybe that was supposed to be Alice's hallucination or something.
EDIT:
https://movieweb.com/friday-the-13th...t-the-villain/