You should consider reading the book Ordinary Men. While it doesn't deal with the men who guarded the camps, it deals with the death squads that were sent through Poland before the camps were established and who were in charge of moving poeple to camps after they were established.

Its been a while since I've read it but the crux of the argument is that thsse were not overly patriotic or cruel men. They had no ambitions to be party leaders and their hate and racism would be about average for the time (not just in Germany, these prejudices were felt in some degree across the western world). They were average men who had been inundated with party propaganda for years. They were often middle aged and unfit to go to the front lines of the military. This was marketed as a way for them to participate in the war and to help their country as best they can. It can almost be compared to how in the United States buttons were made for poeple who wanted to go to war but wefe medically ineligible. It was terrible for those men to believe in the war but be unable to assist. That is how it was sold at least.

The reality of it was that the men were given three to four times as much alcohol ration as a normal squad. In the actual killing, the poeple would be lined up before the pre-dug mass Graves and men would be rotated in to execute. They would fire once then be sent to the back of the line where it might take another half hour before they were brought back to the front. During this time they would drink more. Often, if one soldier somehow didn't show up as often as he should during the rounds there was no punishment. It was grueling and mentally destructive. There were many suicides and desertion was much higher in these groups. The camps were actually started in part because these groups were failing to keep it together. It was too much for them to find poeple, drag them out of their homes, kill them and bury them. It had to be broken up into parts so that no one soldier felt the full weight of the crime.

So when we get to the camps, one man has taken the victim from their home, another has placed them in a ghetto, another put them on a train, another stripped them of all their belongings, another guarded them in the camp, and another pushed the button to release the gas. Your question pertains mostly to the last two men and how they could do their jobs. True, some of these poeple would have been sadistic, enjoying what they do, relishing the power, but there just isn't that many poeple in the world who are sadistic on that level. The reason they could get ordinary men to go that far was the steps they took to dehumanize the victims before they got there.

Often, the victim would have been stripped of everything valuable and everything personal long before. They had no items or clothing by which to show their individuality and humanity. They had often been mistreated or lived in a ghetto and underfed for months or years beforehand so they were emaciated, malnourished, dirty, and afraid before they got to the camp. The guards had long been trained to view these victims as the "other", to see bigger noses or curly hair or whatever markers of "otherhood" would single them out. So when these poeple got off the train in dirty rags with no possessions looking sick, and having (or seeming to have) those characteristics, their brains allowed them to believe that they were not poeple. They were not real, living, sentient humans like the guards and their families. Their brains knew the job they had to do and it could not be done if they believed these poeple were their equals, so they could not. It was made all the easier by the division of labor in ruining their lives.

It was also easy to justify inaction when the labor was divided, and its why you see most 'escapes' or assistance given at the earlier levels of the process. A man who sees the soon-to-be-victim in their own home, surrounded by their family and history, is more likely to help them escape or hide themselves, and more likely to be successful, since he is the one who is supposed to send them on to the next level. When you get to the ghettos, a guard may render some assistance but if he helps someone escape, he knows they will just be found and captured again, and now he will be in trouble as well. Still though, families are still together and they were able to bring some items of heritage and history, they are still human. Once they get to the trains though its a different story. As soon as they arrive they are stripped of everything. They are also far into the woods. A guard who may still see them as poeple and want to help will be heavily dissuaded. Y the fact that they will almost certainly be caught quickly, they are already to sick and weak to run far or fast, and the guard will be found out without issue and punished profusely.

In short, between the dehumanization, the division of labor, and the poor chance of meaningful impact in the event of (small acts of) rebellion, the guards were able to do what they did because they believed it was their only option. They were not monsters, they were not sadists. They were ordinary men doing their job for their country. This is what poeple are referring to when they say that "those who don't know history are doomed to repeat it". If you look for the sadists in your community and say "as long as they don't get power we will be fine" then you will never see how ordinary poeple following orders can commit atrocities. Every person must stand up for what they believe in at every level. Cruelty is the real 'slippery slope' and it doesn't take a monster to act monstrous.