My first reaction is to think that perhaps the behavior you describe and that behavior in humans is the result of surplus supply of prey. In the wild, prey are difficult to find and so they are eaten when killed. When there are more prey than can be eaten, the drive to play and gain hunting experience, becomes killing for sport and a display of suitability for mating.
Second reaction... If there is a range of moral "quality" so to speak of decisions made by humans, and animals also exhibit morality in decision making, then why might there not also be a range of moral quality in animal decisions. Perhaps your murderous chihuahua was on the morally depraved end of that spectrum...
My first reaction is to think that perhaps the behavior you describe and that behavior in humans is the result of surplus supply of prey. In the wild, prey are difficult to find and so they are eaten when killed. When there are more prey than can be eaten, the drive to play and gain hunting experience, becomes killing for sport and a display of suitability for mating.