I lend more credit to the Hannah Arendt's 'banality of evil' when it comes to judgement of people. Or Sholzenitzin's 'dividing line that cross through each of our hearts'. I doubt evil exists in vacuum.
I agree there are many people who would never cross some lines of "evil behaviour" because of a feeling in their hearts, but in my opinion, this is very often or even mostly a question of your education and your environment.
Such an education or environment could also be completely different in different times and in different cultures.
Even in our democratic societies, there were some "experiments" which showed you how easy it is for many, many people to cross some of these lines:
en.wikipedia.org
We Germans were somehow "historically really lucky" to be defeated because of our cruel history because it prevented us - up to now - from repeating historical mistakes which occurred later in other countries and which are happening right now again.
In 1989, the situation in the former communist GDR / DDR could have become similar bloody as in China with the massacre on the Tian-An-Men-Square but somehow, the East-German demonstrators and the policemen and soldiers on the other side there managed most things well.
Although the GDR - state authorities were indoctrinated by communists and their theories, they decided not to shoot on their own people and usually did nothing towards the demonstrators. Hundred-thousands of demonstrators shouted "We are the people!" and by German-communist definition, the police and the soldiers were the "people's police" (Volkspolizei) and the "national people's army" (Nationale Volksarmee). How then could you ever shoot at peaceful people shouting in your own language "We are the people!" ?
Additionally, deep inside, they all also knew the brutality of the history of German Nazism and how it started and where it was leading.
In other countries without such a "national historical shame", the state authorities never had and still do not have any problems to tell their policemen or soldiers: "Go out, shoot and kill the demonstrators!" You could see this in China on the Tian-An-Men-Square in 1989 and you can see this today in Burma/ Myanmar.
Unfortunately, I think, there is no real limit to human cruelties when you have a "criminal state environment" which tells you right from your first day in school that there are "inhuman" criminals on the other side and you must fight against them until one of you is dead.
Right now, the situation at the borders of two states at the Eastern end of Europe makes me shiver again because there are new troops stationed "en masse" and there are really brothers from the same family fighting with heavy arms against each other. One side calls the other "Nazis" and vice versa, the other side calls the one side "Stalinists".
Both do not really regard the other side as still being "human", because simply the use of words of "Nazis" and "Stalinists" in these states exclude their enemies to be still human, seen from the other side.
And this is probably the easiest way how to start cruelties between us "human beings": Describe your possible future enemy as not being human any more and you can torture and kill him much easier than before when you were still regarding him as being "human".