This. As has been pointed out many times in this thread, there's more going on than just the availability of guns. Whether it's "gun culture", contempt for failure, lack of a safety net, too many video games/violent movies, excessive political correctness in parenting, or whatever -- something else is causing people to snap. The availability of guns just makes the snappage a dangerous event.
No, it doesn't exclusively. Anything can be mastered as a deadly weapon and a semi-auto rifle is no different in that it requires more of an investment in training than the media and entertainment have misled the population to believe to be deadly. Anyone who is a firearms hobbyist will tell you this.
I have friends that study Japanese history and many of them are kenjutsukas of decent skill; decent in the absolute scale of mastery relative to samurai. They are more than skilled enough to use a katana or any large knife and cause havoc worse than most of these shooters given the adequate preparation and willingness to die for the "cause". Luckily, they're just harmless video game nerds that would only use lethal force if their lives were in jeopardy.