Yeah it certainly has been a part of our civilization and when you look at that you find extremes. You find people in life or death situations, people who have trained to overcome the barrier to killing, people who are desperate, and people who are just trying to survive. Take war for example, first you train to overcome your inhibitions by repeatedly drilling killing blows. The hope is that you do it almost automatically when you end up in combat. Even the rifle range targets are human shaped. Then when you get there you are in a life or death situation. It's the ultimate prisoner's dilemma. Imagine the most basic scenario it is just you and a soldier from the other army in a room with pistols drawn at each other. You have both been told it is not only legal, but morally preferred for you to shoot the other even if they attempt to run.

Okay but we can all pretend that's moral, what about the Holocaust? One the greatest and most horrible things about humans is our ability to delude ourselves. To believe things that are demonstrably not true. This has helped us achieve space flight where there are several key things that go against what you would assume by looking at them. But it also allows us to look at people and see them as animals instead of people. Dangerous animals even. That's why Germany, and other countries committing similar atrocities use that exact line almost verbatim. Think about how a racist person draws a black person. They always make them look like some kind of animal. That's on purpose, they are de-humanizing them to make violence against them easier.

Finally we come to the last big way to get over the barrier. We all agreed it was better this way. Humans have the capacity to think themselves past the border if they try hard enough, and lock their emotions down. That is not to say you don't feel them, you just aren't letting them control you, freeze you. You're listening to society say this was the agreed upon output of a rigorous process and for (insert ideological reason here) this must be done. Usually the ideological reason is eye for and eye, honor, or because they believe it will dissuade others from doing the same thing.

Alright this has been a lot of blather, how do we know this is true? Well I can offer two studies before I go to bed. In one study they looked at the actual Physical reaction that occurs when the body believes it's going to do harm. I didn't typo, the body has a reaction that is so far down we don't even know it exists without specifically looking at it. The other study was done among combat vets. Specifically veterans who were involved in death they felt responsible for. It turns out if you keep thinking about it you are very likely to get PTSD. The problem of course is you can't just turn it off. You can't wipe it from your memory. It's a damaging thought that's always ready to do more damage.

So there is absolutely a barrier there, an inhibition you have to get over if you want to kill someone.