ISIS claimed it. Some say he was a "leftist angry at right-sided conservatives" and so targeted a country concert (see the CBS exec who was fired for a related opinion). Most accurately, he was a very sick individual who just jumped the line at the gates to Hell.
The world has always been this way. Nothing has changed. This country has a looong history of mass shooting, and the world has always been violent. It may not seem like it, but these are some of the most peaceful times in history.
