Generally I can't recommend this guy, but i think this might be relevant.
TL:DW Historically and in observations of animals not in a predator/prey relationship people (and creatures) don't want to kill.
Post WWII less the 25% of soldiers shot at the enemy and less then 2% "Shot to kill". Because of this the US began training soldiers on human image targets and that rating has gone up to ~95% for shoot to kill. However, generally people are evolutionarily and socially disposed to view killing as bad. We have no known mechanism for helping troops deal with the aftereffects. Thus the increasing prevalence of PTSD as opposed to warfare of the past.
The video opines that it's easier to kill someone you are being attacked by (like in olden days with swords and such). Also back then, you didn't necessarily have to kill an opponent, just cut their arm/leg/chest enough to get them to stop fighting. More deaths were hygiene related. Now you have battles at gunfight and rifle distances where they guy at the end of your sights isn't necessarily threatening you directly.
When we talk about Militarization of the police, we need to look beyond the obvious (military uniforms, military weapons, etc) and also consider that we're using military training techniques for police officers. We are using force-on-force, and silhouette or photo targets. This makes officers more effective when killing is necessary. The downside is the danger that the conditioning kicks in when it shouldn't. And we've seen that the conditioning kicks in easier when the opponent is dehumanzied, often by being another ethnicity. This is why it's a big deal that LAPD stopped using black human silhouettes and started using blue ones.
Now consider the other side. This operant conditioning is available to everyone.
Want to be better at shooting people? Play Call of Duty (or any FPS), airsoft, paintball. I'm not saying modern media and combat sports are *responsible* for killers. I'm saying they help break down the evolutionary and cultural barriers to killing. Modern media makes killing seem like a reasonable solution to your problem. Think back to how many times people have yelled at 2 people fighting on TV "Just kill him!". As we get more removed, and see more killings without consequence, we are creating a new social standard.
As we absorb images of violence and killing as "normal" for protagonists like Walter White, Rick Grimes, the Cast of Game of Thrones, or Jax Teller and his band of merry murderers we are learning. We are learning that sometimes killing is the answer, that it is easy, and often without consequence, that we probably won't be caught. And for 99% of people, this doesn't matter. Most people are not put into situations that are congruent with killing being an answer. Most of those who end up in places of plausible use of force are not swayed by media's argument over the evolutionary and other social standards.but for that minute group, on either side of the law, the will to kill is an easier thing to obtain.
Marksmanship is easy to train. Modern guns are ridiculously accurate. Innovations that people point at as being systemic to mass shootings have been available to civilians in the US for more then 80 years. The will to pull the trigger at a human target is a key factor. It's not the hardware that is the root problem, it's the software.