A combination of:
- a highly competitive social model that produces a high amount of losers and alienated people.
- a media culture that glorifies being famous.
- easy availability of guns.
- a tradition of vigilante justice, that can inspire the idea of "getting even" with all those people that bullied you/looked down on you, or whatever it may be that pisses you off.
- lack of mental health care.
- a fairly aggressive ideal of masculinity, that values being tough and dominant (which may only be possible for some types with a gun in their hand).
None of these factors are of course uniquely American, but the combination of them is.
This. It took a combination of factors to get to where we are now as a society -it will take a combination of factors to solve what led to the massacre. We cannot just focus on gun control as the be-all;end-all of the discussion just because it is easiest to focus on.