It isn’t that Mr. Mittens actually is psychotic, killing birds is simply natural for him. It’s that Mr. Mittens is so angry that someone would question his sanity. Call me crazy and I’ll laugh you off. I imagine actual crazy people go, well, crazy when questioned. That’s Mr. Mittens.
HOBO: Why world would you do that?
MITTENS: I read that cats brings their owners small dead animals. So I went to the pet store, bought some parakeets and smothered them on the way home.
HOBO: You're psychotic.
MITTENS: If you really believed that then you'd know it wasn't my fault because I'm sick and can't help it. If you don't really believe it, then you're just being mean and calling me names.