I saw this on the IFC channel 7 or 8 yeas ago before they started doing commercial breaks during the movies.

Lonely guy lives with his mom in the city, his hobby is spying on people in the building across the street, one woman in particular, she’s way out of his league but eventually they start dating, he’s really paranoid and keeps spying on her, it gets weirder and weirder, he starts to think she kills and eats people, we’re not sure if he’s crazy and seeing things or its real, climax is her having a cannibal dinner party and him going to confront them.