The screen door and the heater (spoilers)
Why didn't Grainey try and find out what was making his screen door open and slam shut? Rather than chop it up with an axe, I think anyone would have examined that thing up one side and down the other... and in doing so would have found the motor or coil or whatever it was, and then there would have been no movie. Same thing with the heater. Since it was so obviously on the blink, why didn't he have a repairman out to fix it? Well, because the repairman would have discovered the device causing the issue... and then there would have been no movie.
It bugs me when a character (or characters) in a movie are required to act in a way that contradicts human nature in order to sustain a wobbly narrative. In this instance, perhaps Grainey's decidedly curious behavior can be explained away by his apathetic state of mind, but it all just seemed to neat, too contrived and cooked-up.
As for the rest... the film had some decent moments of suspense, a solid performance by James Caan, and it did hold my interest. But it was fatally compromised by its sheer unbelievability, the two thoroughly repellent main characters, and one miserable downer of an ending.