The knife as a phallus -- I was expecting that to come up.
Yes, it's small, but the analogy of a bladed object to a phallus is almost as old as that of spears and pillars. It's a, um, anthropology thing, or something. The basic idea is this: Anything that's long and round or that can pierce is phallic in nature (makes you wonder about all those warmongers and gun collectors out there). A spear is obvious enough, but it also follows from that theory that a sword is phallic as well and in turn any type of blade. Now, slashing with a blade isn't very phallic, but stabbing is and all too often a sword or blade is depicted as being thrust into someone. Phallic nature. Consider the legend of how Japan came to be (no pun intended). Izunagi thrust his sword into the ocean repeatedly, turning up foam and that foam became the islands of Japan. Hell, just the description sounds sexual. Consider the legend of Michael guarding Eden. He's a giant, flaming sword. Personally, I think that's the story of the first STD. Think about it. Anyway, if you look at a lot of myths and legends, especially creation myths, if you replace objects in them with penises or vaginas, they start to make some sense. In the case of the slasher flick, it's a bit of a reversal. Life is created through sex; life, or rather peace and happiness, is destroyed through sex. Whether you agree with it or not, that's just what the idea is.