I think it's down to culture and upbringing - I view a lot of our ethical improvement down to rising living standards and higher life expectancy.
i.e. if I might die any day, what do I care if somebody else dies? If all I know is a dog-eat-dog world then maybe I only care about being top dog (or at least am desperate to not be the one on the bottom).
When we have a bit of safety and security to enable us to think beyond our immediate survival, we have much more opportunity to think about others needs.
However, we do know that historically it was a pain in the butt to fill the job of executioner and/or torturer. Quite often they had to take brutal murderers sentenced to death and give them a choice like "either you work for us as the executioner, or we bring over the guy from the next town and hang you". E.g., many outlaw gangs were executed by the first in the gang who took that offer, because the earldom was lacking an executioner.
So the idea that people cared that much less about each other, kinda doesn't compute. If that were the case, people would be lining up to take the job. I mean, you get paid and housed, you don't have to risk your life in a war or hunting bandits yourself, and it's a job you don't have to put 16 hours a day into. What's not to like, right? For the millions of second sons of yeomen, that got kicked out in the street when the older brother inherited the farm, it would be the easiest job to take.
But that didn't happen.
Also we know that occasionally an executioner would break down and be unable to continue. E.g., in the massacre of Stockholm, the guy who had to chop off head after head, at some point basically broke into tears and refused to continue, although this got him executed too.
So it seems to me like the mirror neurons worked back then just as well as they do now.
But when you let sociopaths backstab their way to the top, and hire other sociopaths as henchmen, and there's nothing to keep them in check, yeah, pretty gruesome stuff is bound to happen.