Revealing mistake: During the scene in which Lou forces himself on Kim in the lunchroom, donning a black ski mask, Kim's brother Alex jumps up to his sister's defense. Just before Alex throws his first punch, Kim screams "Michael," referring to the actor's real name Michael Tough. Kim then correctly screams "Alex," the name of the character he was playing in the film.
Visible crew/equipment: When Mr. Hammond drives up to the school where his daughter died, you can see the reflection of the microphone in the left side of Mr. Hammond's car.
Visible crew/equipment: When Kim is in the empty gym dancing, Wendy shows up to confront her. When the camera is on Kim you can see the boom mic coming from the bottom of the screen.
Visible crew/equipment: Kelly gets spooked and heads to the door to check to see if anyone is around. She proceeds to open the front door and the scene quickly cuts to an exterior shot of her looking out the door, and right above the door is a black boom microphone.
Continuity mistake: Kim is at the kitchen table with her brother Alex. Kim has a necklace on. Kim asks Alex about having a date for the prom and all that is around her neck is a gold chain. The shot then cuts to Alex and then back to Kim and now there is a gold star hanging from her necklace.
Continuity mistake: When Robin lays there after she dies her head's facing forward, a shot later her head's turned to her left.
Revealing mistake: A close-up shot of Robin shows she's faintly breathing when she's supposedly dead.
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.