What is it in the American psyche that causes this massively high rate of gun crime?
Yes, I think that is an important point to consider (aside from the firearms question in general) there seems to have been a strange dichotomy in the socio-economic structure within the US for some considerable time - black/white, native American/white, Hispanic/white etc., have/have-nots, right/left, North/South, dope heads/non-users and so on.
This, to me at least, has provided the ground upon which people have been able to ally their fears or act upon their inclinations or impulse by using firearms either in an offensive or defensive manner.
The fact that firearms are so readily available (for the most part) simply provides the means by which a people already at war with themselves can take that one further step towards reinforcing their point of view.
It's strange that many in such a sophisticated country can be driven by such base and simplistic motivators.
This particular incident lends little to the suggestion by some of the left-wing press that extreme right-wing politics within the US was partly responsible for this incident - far from it in my view, I think the basic cause is the availability of firearms to an immature population (in the general sense of political/social awareness) more inclined to acting on impulse rather than pragmatism.