Not to rain on the parade (because this is a pretty awesome thread) but in all seriousness, I toss Rule Zero into the ring when silliness comes into effect. It simply doesn't happen. But if I were to entertain it...
Does it make sense for an intelligent, inexplicably evil housecat to murder its owner? Okay, why not. Cats don't do much anyway, but since this one's intelligent, I'm sure it has something that resembles a motive (maybe he saw the bottom of his food dish more than once this week).
A random, un-templated housecat killing a level 1 commoner? No. Not via damage, at any rate. Infected scratch? If it's good enough for Dothraki warlords, sure! But that's also assuming the commoner fails to do anything about the wound and/or infection, whether by ignorance or lack of means.
Sometimes "abusing" silly things like this brings out some really awesome ideas though. I think there is some genuine merit in using domesticated cats en masse as a disease vector, especially if there is a druid PC. Could be very interesting.