Anyway, just wondering what your thoughts are.
My thoughts are somewhat in alignment with yours. Only exception is that for some reason too many people tend to classify or put every single person who commits a violent crime like that in the same place, in the same category and/or in some kind of political context.
The reasoning for one person to start shooting a church may have nothing to do with why the other guy started shooting at people in a concert, or the other who started shooting people at a church with mostly black people, or the other who started shooting at children in a school, or the other that started shooting at gay men in a dance club.
The act of violence may be the one and only thing that they have in common (aside from access to so many weapons).
I think people around them ignore the signs. Mental health and prevention of violence are never things that people seem to care about. There's no way that people just start shooting others without having some kind of background story, being like the ones that you mention in your post, or other. Whatever it is, that kind of violence is not going to stop for as long as people continue neglecting addressing people's mental health, being PTSD, Narcissistic Personality Disorder, Extreme Hatred for other Races/Cultures, Bipolar Disorder, or Internalized Homophobia.
And then there are the ones who don't have any mental health issue, but are convinced that they are entitled to commit such acts. We, coming from an organization that capitalizes in brainwashing and negative influence, should know well how many stupid things people can do under the negative influence of other(s). Some of those people are perfectly functional and in their right mind, just convinced that it's ok to kill people.