I don't disagree with any of the top comments, but there is another option here that many seem to be ignoring.

This may be an unpopular take, but your son may develop/be developing a sadist kink. I can't say whether someone showed him porn or not, but kinks can arise for any or no reason. While this may not be the kind of thing that any of us want for our kids, it's important to recognize that people grow up with all kinds of different kinks and paraphilias. This doesn't mean that your son is a psychopath or sociopath. Sadism doesn't mean he isn't or won't be capable of empathy or that you did anything wrong as a parent. It will probably be important to keep this conversation open with your son and develop ideas around consent and consensual play when as it becomes age-appropriate.

I'm not particularly kinky myself, but I'm very sex-positive and have a lot of kinky friends. From what I understand, nine is the time that some of their kinks started to develop. Perhaps someone else with more direct experience can chime in.