It didn't matter that it was so easy to guess the killer
The movie wasn't about the killer; the killer was simply a device used to show that Spence had his own oddity. The movie was called Good Neighbors, because that's what the movie was about -- the interaction between these odd, strange, and quite bizarre people. The movie's purpose was to show that on the surface you can have good neighbors, but just scratch that surface a bit, and you find out that they're all nucking futs. The serial killer aspect was not the story in the least.
That being said, the movie still sucked, and I say that because none of the characters were likeable in the slightest, so I couldn't have cared less what happened to them. Well, maybe Victor was. We tend to feel a bit sorry for socially awkward, introverted geeks who are totally out of touch with reality. Nah, even he wasn't likeable now that I think of it. He was pitiful.
I liked the landlady. It would have been a hoot had she done ALL the killings in the movie!
Also, I was somewhat glad that the Langlois bitch got her comeuppance, not just for the cat murders, but for being a general twunt all the way around.
Don't let your mind wander. It's far too small to be out by itself.