Well, the dramatic tension - is their neighbor a killer, or isn't he - sort of drives the movie. Remember it's clearly a homage to "Rear Window", and the murder in that movie was crucial to the key's plot, not an incidental event.

Well that's the whole point of the movie in the first place. It's not entirely implausible. People find out crazy things about their neighbors when they really pay attention. For example, it was discovered that neighbors down the street from me made porn in their own home.

I disagree. If it weren't for that part of the movie, I would never have rewatched it. It's a nice payoff.