They are just mad, mad, mad. Don't feel their life is worth anything so they can't imagine another's life being worth anything either. Their compassion setting is shut off and they don't see humans or fur babies as anything but insects.
Like stepping on an ant or swatting a fly, they annoy us, we kill them, feel good about getting that damn fly and move on. That damn fly I am so mad at it, I will kill the SOB. That is how they see others.
The question really is HOW did they get that way? Were they born defective? Or made so by socity? Either way they kill. Babies can be born defective in body and mind.