Continuity mistake: When Margaret has a knife thrown in her hand, in the first shot, there's no blood, but in the very next shot, there is.
Continuity mistake: When Margaret is first stabbed in the chest with a kitchen utensil, in one shot there's no blood in the wound, but in the very next shot there is.
Continuity mistake: In the scene in the beginning of the movie when Carrie's mother puts her in her "praying room" the room is pitch black. When it cuts to an overhead shot of Carrie in the room, the room is lit either by a window or something else. This is before she lights a candle.
Continuity mistake: Before Carrie goes to the prom while her mom is lying on the bed, her head changes position between shots.
Answer: He starts to genuinely like her, hence his disgusted reaction at the sick practical joke played on her.
Manky