My 9 year old son bit the head off of his gerbil. What is wrong with him?

Last night, I witnessed my 19 year old son biting the head off of his pet gerbil. We've had a few incidents with him hurting animals before- two summers ago, he got mad at me and threw my cat into the swimming pool, and the year before that, after a huge fight with his father, he threw our beloved dog, Charlie, into the fireplace.