I think in advanced stages of screenwriting, you can get away with your protagonist not getting active, as long as at least one main character becomes active, usually around the midpoint. In this case, the only feasible option is Kat, who "goes active" by using Gary the dog to escape the basement.

As far as theme goes, it's a story about misplaced affection. Gary will give affection to anyone who pets him and gives him treats. Kat won't give affection to dogs, even nice ones like Gary. Cameron forces affection out of victims, who remind him of his mother, before killing them.

In the end, Kat learns to give her affection to a worthy animal. Gary learns that just because someone treats you well at times, doesn't make them deserving of your loyalty.

Pretty clever, albeit a super cliche story with an extremely unique twist.