When you are dehumanized, you act less upon personal instinct but based on a sort of hive mind. It is also easier to commit crimes and atrocities when you are in uniform vs when not in uniform.
Phillip Zimbardo (professor and the man who ran the Stanford Prison Experiment) goes into detail how putting on a uniform, wearing a mask/face paint, helps people to rationalize committing crimes and doing things they would deem immoral otherwise.
I don’t know if you can call it intentional evolution of dehumanization over time but it has definitely evolved. You can see in the video soldiers committing multiple war crimes and often many of them are wearing a disguise over their face.