The same thing happened to the Nazi einsatzgruppen during the opening stages of their genocide(s). Even with a lifetime of propaganda conditioning one to see the enemy as subhuman untermensch terrorists the average human psyche cannot cope with the reality of committing mass murder. It’s one of the reasons why the nazis sought a “Final Solution”, they were unable to kill as many people as they wanted and their death squads were falling apart due to drug abuse and psychological trauma, so they wanted a method of mass murder that gave more distance to the act of killing. Gas chambers provided that