I'm not sure I agree with your comment. While I would be willing to concede that witnessing the visualization of violence might have some kind of affect towards increased desensitization, that's not necessarily a negative.

In some situations being able to keep a cool head in a crisis or in an area where something like this might occur could actually prove to be a benefit, as you are less likely to enter a panicked state (such as working in an environment where you may witness excessive amounts of blood/gore, eg paramedic)

I think that the vast majority of people (teenagers included) are more robust than we give them credit for, and can distinguish fantasy from reality.