Who was the killer in the original Friday the 13th?

Pamela Voorhees was the killer.

extract.

"Pamela Voorhees (nee Sue, born. 1930 - died. June 13, 1979) was a serial killer and the main antagonist of the first Friday the 13th film and remaining a major posthumous antagonist for the rest of the series. In the original film, known simply as Mrs. Voorhees, she was the mother of Jason Voorhees, whose drowning death as a child drove her to murder those who she deemed responsible."

https://villains.fandom.com/wiki/Pamela_Voorhees

The answer can be found in Entertainment & Music, Movies. Which ISN'T here.

It's this guy but he's wearing a mask so it could be anyone really. It looks like the mask Hannibal Lecteur wears but I'm not sure it's the same person underneath it.

Jason's mom. It's confusing until you see the whole movie.

Mrs. Voorhees.

I don't believe they ever gave her first name until one of the sequels.  Which was Pamela.