HELP ME find the name of this film! (killer neighbor)

Hello, i watched this film in the early 2000s, its about a girl who moves in to a new apartment, and the guy in the apartment she can see through her window is attractive, thing is she starts suspecting hes a killer. Theres this one specific scene in which he brings in a girl to his apartment, pulls out a knife but instead of killing the girl he kills a fly. This scares the female lead, but makes her doubt he is a killer. But then days later (?) she caughts him with a bag that looks like it has a corpse, she follows him and takes the bag, opens it, but theres only toys for kids, cause turns out hes a “nice” guy who plays with kids. So she stops suspecting him and starts dating him, i dont remember more but i think he did actually kill someone and hes actually a killer.

thank you god bless you

Head Over Heels (2001)