Scale, things in America are bigger, contrasted with ireland's 6 million population, we'd expect to wait 50 years before seeing an event like this IF you see 1 per year in America.
But that isn't the whole story, scale amplifies some effects, so it is possible to develop a culture with 100,000 members supporting one-an-other whereas in Ireland that would be 2,000 people who might have less support or extreme views, and thus meet each other (by chance) less often, and reinforce their extreme views less often.
This at least is my explanation for groups like the Westborough Baptist Church, as an example of extreme cultural variations. This can of course have positive extremes aswell, but your example is not a positive one.
How many 10 year olds are perfectly well and healthy? and what does health (mental, physical, emotional etc.) have to do with a complex system of cultural diseases. The idea that violent video games has no impact on individuals is often put forward, and I would agree, I managed to develop into a healthy adult regardless of the violence in any video games I played, but that doesn't mean all individuals will have the support (parents, family, teachers, and friends) which I had. TV, news reporting, glorification of warriors, all kinds of cultural factors (which are each amplified in the US, bigger and more extreme) each influence an individuals chances of developing to be a 'healthy' adult.
I don't think you can simply blame parent's or video games (there is violence in the news and in the streets) can account for this changes. Urbanization tends to increase the levels of stress and depression in individuals, illnesses of civilization where the only difference is who in your community is there to support you - this could be seen as a illness, a societal level illness, but it is far from simple. No one factor is to blame.
Also, who is Maple? i haven't seen anyone by that name around for some time...