I prefer to think that I could never turn into a monster. Solzhenitsyn managed to hold on to his humanity, through some of the worst atrocities imaginable.
"The little daughter's on the mattress,
Dead. How many have been on it
A platoon, a company perhaps?
A girl's been turned into a woman,
A woman turned into a corpse.
It's all come down to simple phrases:
Do not forget! Do not forgive!
Blood for blood! A tooth for a tooth!"
-Prussian Nights by Aleksandr Solzhenitsyn (https://en.wikipedia.org/wiki/Prussian_Nights)
You would be surprised. Some of the most terrible people in the Nazi concentration camps were fellow prisoners tasked with being Kapos (prisoner overseers). The Kapos were spared some of the worst labor but in exchange had to show extreme brutality to the prisoners they oversaw so the Nazis wouldn't question their effectiveness.
This isn't the only example: Stanford prison experiment and the Milgram experiment. I'm sure there are others. When you value being alive many people will do anything to survive.
"The little daughter's on the mattress, Dead. How many have been on it A platoon, a company perhaps? A girl's been turned into a woman, A woman turned into a corpse. It's all come down to simple phrases: Do not forget! Do not forgive! Blood for blood! A tooth for a tooth!" -Prussian Nights by Aleksandr Solzhenitsyn (https://en.wikipedia.org/wiki/Prussian_Nights)