There's numerous issues here. I'll try to explain my perspective as a list.
1. There has been many trying to connect violence and video games, yet each experiment assumes that voilent videogames create a violent person. They do not explore if a violent person is attracted to videogames.
2. It takes away the personal responsibility of the individual commiting acts of violence.
3. The media hype surrounding violent videogames has unjustly associated violence itself with computer games, despite there being no definitive proof in any situation (i.e. The media jump on the bandwagon of blaming GTA).
4. If you notice, only guns are involved in the effects of computer games. Why don't we see more people breaking their heads on brick walls "because Mario can do it"?
5. Those games were Mature ratings. Why are a) the parent's letting him play it and b) the parent's bought it for their child/the retailer allowed a teenager get hold of a mature title?
6. Most consoles today have an in-built parental lock on them.
7. The media effects theory has always been rought with criticisms.
8. The kinds of people trying to "protect" children from this are the "moral right". I know one of the biggest lawyers involved in this kind of case - Jack Thompson whom wishes not only to "protect the children", but also to take down Take-Two, thus meaning nobody, no matter how old you are could play their games.
9. Taking a single medium out of context and blaming it for something is not a good way to determine something. The single-cause idea denies the possibility that numerous factors, including other mediums, social situations and Global influences may have contributed to the killings.
10. It demonises a sub-culture. Whereas in fact, the number of people doing these killings are significantly smaller than those who have bought/play the game/s.
11. It implies that one's own perceptions of reality can be distorted to the extent whereby one cannot distinguish between fantasy and reality - it means that these people were under the influence of games.
12. These problems are mostly associated with America/Canada. Therefore, there's got to be a factor relevant only to the US/Canada that other countries do not have. (If it really was the fault of the computer game, we would see shootings happening all around the globe, not isolated a few particular countries).
Anyway, that's all from the top of my head there.