There are too many factors in play to pigeon hole videos games. Millions of people play video games...far more than shoot regularly. The problem is, most mass murderers fall under the same basic profile. Anti-social loaner with serious, either un-diagnosed, or improperly treated mental issues. Video games provide an outlet for anti-social loaners because they can "interact" without being social. I think the fact that this group of people tend to play video games isn't indicative of video games being part of the problem (as in, causing people to commit mass murder) but rather it's a symptom. These type of people are attracted to video games because they allow "social" interaction without actually socializing.
Repeat after me:
Correlation is not causation.
We need to figure out why there's a correlation before we can say it's part of the cause. I don't think video games caused these problems...they're simply something these types of people are attracted to. There were many mass murders before violent video games...