Simple... Nothing "magical" about it. More like "scientific".
Stress, Stimulus, Response, Reward - repeat as necessary.
Repetitive, autonomic, gratuitous, and extremely graphic violence depicted in a casual, "entertaining" manner affects the "Limbic Brain".
It's called "Operant Conditioning". It's very much related to the techniques we use to train (actually, to "condition") our military, police, and high-end security forces to kill.
See the work of LtCol. Dave Grossman. He's one of the world's leading experts in the field.
http://www.killology.com/
http://www.killology.com/book_stop_summary.htm
http://www.killology.com/new_media_vio.htm
http://www.killology.com/killrev.htm
I've spent a significant portion of my professional life using far less sophisticated simulations to train armed professionals in the application of deadly force. That was under tightly controlled conditions, and it works very well. If we can use them to "train" mentally thoughened and prepared warriors, what's the effect on a 13-year old kid who might have a host of other problems, and no boundaries.
We all like to pretend that we are individually "too smart/tough/mentally alert for it to affect us, but if you can accept the premise that media imagery affects people's consumer behaviors (one kind of has to - Madison Avenue spends over 100 billion dollars a year on that bet, and they win every time), and other behaviors (how do you feel about the psychologocal effects of pornography?) you have to accept the premise that almost any other behavior can be affected and/or manipulated with similar techniques.
Again - it's simple... The real question is: If someone spends a significant portion of their lives playing violent, angry games on screen, why would anyone be surprised when they turn out to be violent, angry people.
I think the key is “host of other problems and no boundaries”. The fact is MILLIONS of people have been playing these violent video games for decades now and the only time anybody goes buggy it turns out they were already buggy.
Advertising works completely differently than what people are claiming video games do to people. Advertising pushes product awareness and bandwagoning. Neither of which are even slightly applicable to this, we already know about violence, and there is no bandwagon for it.