
In real life, it says more about a dog's training as many owners of small dogs continue to treat them as puppies rather than full grown dogs who need to be disciplined. When they are genuinely annoying, this characteristic is given to stereotypical small yappy breeds that reflect their owners. Unlike cats, dogs are rarely portrayed as evil unless the cast specifically only features dogs and antagonists are needed (the obvious exceptions, of course, being the tropes of the evil tear-'em-to-pieces junkyard dog, the psychotic poodle and the Hellhound).

Nor is the fact that a whole category of dogs - terriers - also kill small, cute rodents. Notably, the real threat that mice present - their ability to overpopulate, consume stores, and carry dangerous parasites, which is the original reason we bred cats in the first place - is seldom mentioned in fiction. Protagonist cats rarely eat mice ( rats, on the other hand.).

Since mice (and birds) are often depicted as being intelligent, the express desire to eat them becomes a type of cannibalism and is therefore evil. Dogs aren't exempt from this behavior in real life but are rarely portrayed as predatory in fiction. It certainly doesn't help considering highly-marketable, small creatures are typically the kind of things cats see as prey. As a result, many writers who like cats, such as the late Terry Pratchett and Paul Gallico, play into the trope by presenting their pet as something of The Chessmaster, expertly manipulating humans.

Parts of this have to do with traditional traits that even cat lovers admire - independence and pride for some equals lack of love for the owners and aloofness for others. Both sets of animals will have vices, but a dog is more likely to do harm unintentionally. While cat and dog owners can cite a truckload of quirks on both ends of the spectrum, when both species are featured in fiction, you are far more likely to find an outright cruel, nasty, and otherwise vicious cat character.
