Crazy isn't the variable here; it's human density. We've ALWAYS had crazy people. We've ALWAYS had people who decided to go on a rampage of one kind or another for one reason or another. What we didn't always have were so many people, so close together, with the tools to kill so many people at one time -- and 24-hour access to all of the information in the world whether you want it or not. We have a greater mass of people crammed into an increasingly small habitable volume. Some of those people are going to be psychotic killers. The difference between now and say, 1712, 1812, or 1912 is that back then psychos had fewer easily-accessible targets, less efficient tools to kill with, and no all-pervasive media to tell anyone about it when they did.