Bay of Blood (a.k.a. Twitch of the Death Nerve 1971/Mario Bava) is generally considered to be the first "slasher" (or "bodycount movie") that set the standards for the likes of
Friday the 13th and
Halloween to follow.
Many death set-pieces from it were directly ripped off for Friday 13th Part 2, like the axe-in-the-face and spearing sequences.