Question: Were there any hints of foreshadowing of Alex being the killer?
Question: Why in the world is Alex, when unmasked at the end, wearing lipstick? Symbolic or something?
Answer: The film doesn't provide an answer (which I think is a good thing). My interpretation is that Alex has somehow absorbed his sister's spirit, symbolically (not literally), and is avenging her death *as* Robin, in a way. Her name is his last word before he dies. A scene was shot but cut which revealed that Robin and Alex were twins. That scene was added back to the television edit of Prom Night.
Question: I understand why the killer was after the 4 teenagers that were responsible for Robin's death. But there's something I can't figure out. The girl that was with Slick and having sex with him in his van, she wasn't one of the 4 teens. Neither was Slick. First he killed the pigtails girl, she was there. And then he went after the girl and Slick in the van, but neither of them had connection to Robin's death that I could figure out. He then went after the girl in the red dress who was one of the kids, and then went after the other two, the guy and the main girl. But Slick and the girl he was having sex with in his van, I can't figure out why the killer left the school to go after them. Why?
Chosen answer: The girl in the van was Jude. She was involved in Robin's death. You see her flashback as young Jude when she receives her prank phone call then she meets Slick walking on her way to school.
Answer: Alex is not the typical one dimensional slasher serial killer, but rather a complex individual with conflicting motivations. The lipstick is a symbol that he is portraying the hurtful people from his past even as he kills them.