In Animal Farm, why do the dogs try to kill the rats?

The rats are first considered outsiders and enemies since they are wild instead of domesticated. But then the farm animals put it to a vote whether the wild animals are to be considered comrades or not. The vote falls in favour of wild animals, which are also to be considered comrades since they go upon four feet. (At this point the chickens protest until they are told that their wings are appendages of propulsion and therefore are under the category of 'feet.')

The role of the dogs here is a foreshadowing of their later repressive role under the tyrannical dictatorship of Napoleon, accompanied by political purges and executions.