Luckily, actress Anna Torv, who helped bring some dramatic gravitas to the kittie scenes, had her own explanation, which she provided in an interview with TVLine. "I thought, ‘This little kitten is representative of all these faceless [victims] and we only notice the ones that are dead because they have families that are looking for them,'" she explained. "And then here’s this little abandoned cat that no one is going to care about. And if that was a person, it’d be the same thing."
According to the same interview, she passed this theory along to David Fincher, and he had a different read on the feline interaction. He thought the scenes meant that there was a child in the building going around killing cats and that there was a young sociopath in Wendy's apartment building. Will this cat murderer eventually become a case that the FBI investigates? We'll have to wait for Season 2 to find out.