A mark of a good proto-slasher is a big creepy 👁️ peering out of the darkness.
You've got the horrible 👁️ in Black Christmas, it's a common motif in Peeping Tom, and let Deep Red synecdochically represent gialli in general: there's an incredible one there too. It's a way of hiding the killer's identity until the end of the movie while also distilling him to predation and voyeurism. When Halloween and Friday the 13th spawned a torrent of imitators that…