It's been awhile since i've seen the movie, however, In a scene early in the movie Pullman and kidman noticed that the neighbor kid while playing piano in his room could see them through the window in their bedroom.

This made Pullman think later that to trap kidman he would make her think the kid had seen her and baldwin together.

I think that was the reason...i thought it was a decent movie...didn't Gwyneth Paltrow play the college girl who was murdered?