Comparing violence in video games or movies and real life is ridiculous. A competent and sane person would not follow this trend to becoming a mass shooter just because of a virtual world emphasizing violence. If you take this logic, and apply it to a different type of video game, such as city building simulator or Guitar Hero; sure these video games are enjoyable, but that does not give you the skill or the means of becoming a real-life city planner or a professional guitarist.
Additionally, a good majority of teenagers are playing video games like the Call of Duty or Battlefield franchises, and a majority of teenagers are not violent in this particular manner. It is later found that the people who commit these mass shootings have had mental disabilities and troubles in the past. And sure these criminals have been said to have played violent video games, but again the majority of people who play violent video games are not violent.
All I ask for people to keep in mind is correlation does not equal causation