I can't remember where I read it, but years and years ago when researching serial killers and their "early years" and seeing so many similarities to killing animals at a young age, one mental health professional wrote that for children that experience extreme abuse, the desire to hurt or kill small animals (or even other kids, younger than them) was their way of taking back control of their own lives.
In a very dark, sad, and twisty way, that totally made sense to me. When an adult severely abuses a child, it's a VERY good bet they might be turning that child into a monster in their later years.