I think it's a leap to say watching




will lead to an addiction to




and/or will then lead to violence. Ditto video games and every other thing that gets brought up as people try to make sense of that which makes no sense. Does it happen
sometimes? Yes. It happens in individuals who are predisposed to being violent anyway. If not one thing it would be another.
Everyone wants to know why, and wants to point a finger somewhere. It has to be someone's fault, just has to! But there are no simple or pat answers. You can't make sense of something like this. This kid has brain wiring and a structure and environment around him that created a perfect storm of evil. He is someone who will need to be studied by experts. It will never "make sense." I don't know how something like this can be prevented because most parents, most of the time, think their sons and daughters are wonderful and good and cannot conceive of this--that comes up over and over in cases where the perp is under 21.
As an aside, if this 17 year old were a murder victim instead of the perp, he'd be referred to as a "child" or a "baby" by many folks. I personally find that ironic.