Here are my thoughts, especially when my above post link points out about America have the most shootings,etc.
There are some unique variables we have that likely contribute to it.
Aside from the fact the easy availability of guns and semis.....
1. Mental meds - This one is huge yet so silent I bet most of you will have never heard or thought of this before. Many meds being prescribed, depression meds, etc, have DANGEROUS side effects and withdrawal effects! Not only dangerous to the user but to everyone around them. If you do some research on some of them, effexor for example, you can find tons of posts on people getting uncontrollably violent after taking it or withdrawing from it. Many of the others can do the same. Yet they are prescribed like candy. I don't know why after any crime/mass shooting/etc a record isn't kept on the meds they have been taking, when prescribed, when stopped, etc to get solid information. But for those who had no history of violence, this would be one of the first things to look at. Americans are more medicated than any other country.
2. Vacation/Employment law - Most are afraid to take off their meager 2 week vacations each year. Whereas in Europe average 1 to 2 months vacation per year. No doubt stress and no recharge time will contribute to breaking points.