Dozens of countries don't allow the posession of guns and crime rate doesn't skyrocket because of that. Some countries don't even allow policemen to wear guns. Some countries have laws that punish the posession of firearms with death, and I'm not talking about some unknown banana republic, I'm talking about a mainstay of technology and research (let's forget about youtube views) such as South Korea. Yeah, the country that exports phones, cars, all sorts of electronic components, and what not. Those countries don't get those killing sprees at schools.
If videogames played a part there, or what part it was, I can't tell. I personally don't like shooters, unless the experience goes far and beyond what sneaking around and scoring headshots has to offer. To make myself more clear, I'll say that while I enjoyed Borderlands, Halo and Crysis:Warhead, I won't touch CoD of MoH or even poke them with a stick. Halo setting and story got me glued to my chair from start to finish, and I loved the AI, level and monster design. I played Crysis:Warhead to test my (then) high-end machine and also because I found the story appealing - it was short and to the point (which I knew before I started playing it) - and the sci-fi parts. I've never done multiplayer in any of those games though. I've played extensively competitive multiplayer in SF4 and its following iterations though, and I've loved every second of it.
I don't think playing shooters is at the core of the problem. What I do think, though, is that people who spend way too much time on their own, people that don't grow as a person, that isolate themselves from others, can lose the ability to empathise with the world around them. I certainly see that as a problem. That many blockbuster videogrames trivialize violence might not help, but that's not at the core of the issue.
And yes, having guns at every home definitely is a part of the problem, and a huge part at that.