A serious warning sign that the person doing the cruelty has the potential to become a serial killer. Especially when the person is younger in age, such as a teenager or even younger.

Of course, it's not a guarantee, but it means that the person needs some serious help and therapy to make sure they stay inside the bounds of sanity.

Shows an utter and complete lack of empathy, that the person either does not comprehend that other creatures have feelings, or even worse, that the feelings of others are meaningless to them.