That's right, there's something about the deep, dark American psyche that causes otherwise healthy and balanced individuals to turn into cold-blooded mass-murderers...Think about it - have you ever been sad or angry enough to literally want to mow down crowds of people you didn't know? Of course not, you have a conscience and you value life. None of the people that have perpetrated these crimes were particularly stable individuals...
It's all a fairly new phenomenon in the history of the U.S. that simply became the popular thing to do if you're depressed and disturbed. Your handiwork is guaranteed to be all over the news for about a week, along with your picture - in the internet age, you can count on being famous world-wide. These individuals idolize those that came before them and desire to be in the "club" founded at Columbine. When those two first killed all of those people, it broke down the invisible barrier that kept people from believing something like it could actually happen. It was horrible, powerful stuff. That kid in Finland, or the one in Germany are just examples of how this phenomenon is transferable to other cultures.
If you want to blame American culture, you can blame their free society, prosperity and emphasis on the importance of the individual for giving these guys the selfish notions that their feelings are important enough to grant them control over the fates of others.
IMHO