Keep in mind, I'm not in favor of gun confiscation but some of your observations are interesting.Maybe I interject.
Those people are neglected, stipped of any opportunities, usually singled out, devoid of attention, maybe selfcentered, and suffering from an allure of grandiosity, but thats just one factor.
Their mind (subconscious) then looks for a way out. (We are talking severe mental health stuff here.) So they are drawn to power fantasies.
Guns. Make people feel powerful. Just touching one, does (apparently ).
Then the entire cultural stuff america has prepped up around gun ownership and soldier heroes comes in, and also makes some of the difference.
(My understanding concerning that stuff is, that those are necessary myths within your society to deal with or integrate the part of your society, thats out in the world fighting almost neverending battles. Through coping with that over years, your society is structured a little differently, than that of other nations. (If you tell someone just returning from a war to check their weapon at the counter and then play happy family life, shortly thereafter - it usually doesnt work out so well, so you have groups and organisations that take this personality and the weapons they are accustom to - and form little subs societies, that slowly guide the transition into a more civil life.. Something along those lines. Weapon culture is more of a thing in america than in other developed parts of the world, partly because of it - in my opinion.))
Then you've come to realize, that the potential shooter jumps another gap, where the anger he has stored up gets projected towards the people that "did him wrong", or that have the opportunities he'd liked to have. And then he jumps another step, where he acts on it.
None of this develops "silently" - there are warning singns all over the place. People around them are ignoring them.
The motivation then culminates into the following. "No way out" thinking. Powerwish. Anger projected towards peers. Feels power in the form of handling weapons. Has a societal image of death near or related to glory (that warped in their minds). And knows, that they will be important for that day, and maybe that week. Also - punishing the people that hurt them. When the shooting begins, they probably get into a blood frency I dont know - I don't deal with military psychology, I just assume, thats why they pick out victims randomly after the first few.
Now you probably have a decent understanding of whats causing it. Does it help you directly? I'm not sure.
Reducing the numbers of automated weapons around reduces the number of victims. Thats a first step.
Do you know where gun culture really ramped up? Not it's roots in America but where it really became cool? First, in reference to the roots of the gun culture, this nation was liberated, won and founded with guns. After America won her independence from England, guns were still vitaly important. The land was so harsh and so untamed that you needed those firearms almost constantly. It was a time when there was no electricity, no running water and no ready made food. Every man old enough to use a gun, had to use it to kill food to feed his family. Also, there were the natives. That is too deep of a topic to cover here, but every man of fighting age found his gun a necessity to defend his very life and those of his family. They could not just call the police in those days.
Now, as to the time when guns became the object of every starry eyed boy and a few girls too. Was, after our fighting men had helped to assure world peace in the wake of world wars one and two, after they returned, they were national heroes and every young man wanted to be like them. Later, when television programming began, there were shows that further glorified the US GI and also the cowboy shows and movies because in that age, that was what the American public wanted to see.
As time has progressed, television programming has grown steadily more violent, and movies, not to mention games. So it makes sense in a way that, without proper guidance, if you allow television or games to raise your children, this is one possible outcome.