Ask your mother, point blank, if she thinks that you are homicidal. If she says no, tell her that her theory is obviously flawed. Logical Trap, I choose you. You play violent video-games but you're not homicidal, therefore violent video-games do not make a person into a serial killer.
Give her condescending smile and say, "ah, now we're getting somewhere." Ask her if she thinks Jack the Ripper played violent video-games. Ask her if she thinks Charles Manson, Richard Berkowitz, or H. H. Holmes were knocking people off after playing Mass Effect. Ask her if, perhaps, psychopaths might have other factors in common--such as, for instance, being psychopaths.
Ask her how video-games are making our society more violent when violent crime per capita has actually gone down since the rise of video-games. http://kotaku.com/5814601/violent-video-games-contribute-to-dropping-crime-rate-study-suggests
