There is more going on than just incitement, or violent imagery, or mental illness, or anything else cited in the MSM in reference to the recent tragedy.
What happens when more and more people are persuaded that the entire concept of society is a fiction? The government itself is evil? That social institutions no longer protect and only enslave?
In literature and history, we don't need to look far. We know what happens when the thin veneer of civilization is stripped away. What occurs in the absence of civil institutions is illustrated in books and movies, usually with a catastrophe such as an apocalypse (The Postman) or shipwreck (Lord of the Flies) as the backdrop.
Unfortunately, the same type of catastrophe can occur at any time, anywhere. It doesn't have to be real - you just need people who, for whatever reason, think that it is real. If it is real to them, and they act upon it, then it becomes real. Disaster follows.
Thanks to foucaltspendulum for getting me started.
After hundreds of thousands of years of living in small clans or extended families, humans were abruptly (as measured on the scale of time) thrust into a novel circumstance of dealing with thousands, and then millions, and now billions of strangers. Our successes and failures in dealing with this change comprise the story of civiliation as we know it.
Everything institution that we take for granted, every convention that helps us get through the day without the occurrence of violence or hurt (when in fact we do), is a product of amazing, brilliant, and fragile concepts that we have only invented in the past few thousand years. Civil society - humanity's most amazing and miraculous concept. The idea that we can, and should, and ultimately must, live in a fundamentally cooperative manner with both neighbors and strangers.
This in no way means that people of ancient times were not inherently good (doesn't mean they were bad, either). It just means that until a few thousand years ago, the very concept of interacting successfully with people from farther away than over the next hill is something we never got any practice at.
We all have the inherent potential to lose trust, and to act on that lack of trust. If someone broke in to my house this moment and I thought the person was a serious threat to my family, my trust would be gone in an instant and I would have to defend my cave. I have a (now long deceased) family member who held off a squad of marauding Cossacks at the farm house door with a pitchfork to protect her children (really, the ones with a C, not K).
The question is not whether we have that potential - we do. The question is what circumstances may trigger it.
The most insidious thing about rhetoric from the right is something that goes beyond the violence and demonization of specific people. It is the undermining of the very concept of cooperative civilization. Once any person comes to believe that there is in fact no society, no civilization, there is nothing left but to defend that cave in whatever manner seems meaningful.
Stripping the institutions of civil society, the right is working to create a self-fulfilling prophecy: As society fails more people, as the belief erodes, more people will fail society, in some cases violently, in turn creating further justification for more enforcement and less safety net.
Violent images are not the problem in isolation. They are at their most harmfully powerful in the context of a model where there is no more civilization. Someone can like guns, can own guns, all those things, but will not use those guns aggressively except in one circumstance - when that person believes that they are truly on their own.