Here's some first-hand research that's free, won't cost taxpayers 10 million dollars, and where it should stop:
It doesn't matter if I play COD, Halo, Far Cry, AC, Battlefield, GTA, or watch movies that have a body count in the hundreds, because even though I can act like an absolute baddass in these games or watch those that act like it in films - it's not the reality we live in. I, as a sane individual, can distinguish easily reality from the "fairy-tales" created within this violent media that I/we play or watch. The people that clearly are disturbed, on the edge, present themselves as very extreme in one form or another, or however you wish to title them, these are the people that have very high tendencies to snap or act out within due time whether it's out of frustration or attention.
I don't really blame any of the "violent media" because those that don't have a hard time understanding what's legal and what isn't, what's reality and what's not, and what is morally right or wrong - these are the people that's will not act out and the rest of society will have a problem with. There are those that cannot distinguish these differences, do not care, or long for the immortality that a massive event may bring to their name. This is your target group for these incidents, and restrictions can be as numerous as legislators wish to impose on all of the public, but it doesn't offer the solutions to the real problems that these select individuals face within themselves.