@jamie,
"If gun ownership is not the cause of gun violence, then what *is* the cause? What drives American gun owners to kill, where their cousins in other nations manage to exercise restraint?"
I don't think there's a single answer. Most American gun violence happens in the inner city, much of it black people killing other black people. Our inner cities are far more toxic than those in most other countries, and the reasons are extremely complicated. Obviously, our history of slavery and foot-dragging on civil rights for a century is a big component; one can then dig into more detail on the specific steps along the way that transformed those behaviors into urban disaster. (See also: US incarceration rate and that of the world, compared).
Then you have the mass shooters. This is a relatively small blip on the overall homicide rate; we're talking about it because it garners far more news attention, for reasons having to do with what makes exciting news. (Good luck ever seeing the latest gang shooting in Chicago on the national news). Nevertheless, it's still fair to point out that the US is in the upper level of countries when ranked by mass killing deaths per capita. (See http://www.politifact.com/truth-o-meter/statements/2015/jun/22/barack-obama/barack-obama-correct-mass-killings-dont-happen-oth/ ). I think that this particular killing actually acutely highlighted one of the real reasons for this: all the notoriety and attention accorded to the killers, which is extremely appealing to people who feel they are life's losers and will never be heard. So I think it's a kind of cycle that has erupted recently (notice that it has trended sharply upward relatively lately, despite gun culture being an extremely old aspect of American culture).