You want my take on it? Here goes:

When I try to answer this question, I think of the fact that something somewhat similar is happening in US prisons.

• People are often sentenced to prison for small, non-violent crimes (drugs, failing to pay child support...).
  
• Additionally, the justice system is often a travesty as most convicts never get a proper defense that ensures a fair trial.
  
• Prisoners can be forced to perform labor. Some of that labor is used by the prisons to turn a profit, and sometimes that labor is pointless (like digging trenches then filling them up).
  
• Prisoners can be detained in inhumane, unsanitary conditions.
  
• Prisoners can easily be abused by guards and often have little recourse when that happens.
  
• Prisoners can be raped.
  
• Prisoners can be murdered.

Anyway, I realize it's not as bad as what happened in Nazi concentration camps, but there are similarities that should horrify us (just like we were horrified when it happened to the people in concentration camps) and it should make us ACT against this system (beyond merely complaining about it on the internet once in a blue moon). The bottom line is that people in prison are often being abused and have their human rights violated, and many don't even deserve to be incarcerated to begin with (and even those who deserve to be locked up don't deserve to be abused, although this last opinion might differ depending on who you ask).

Yet we don't act. Why?

For the sake of discussion, we can try to imagine that one day in the future a strong military power decides that this inhumane treatment of convicts is criminal and decides to intervene to save them. What will we tell them when they ask us "How come none of you did anything about it? How come police officers didn't quit their jobs instead of being part of this, if they were against it? How come the people living near prisons just went about their daily lives, ignoring the abuse happening behind these walls? How can you say you didn't know?"

Well, I can tell you that the answers we'll give will apply to the Germans during WWII. Here are a few:

• "I didn't really care because these people negatively affected society in the first place (A common thought about Jews in Germany during WWII)."
  
• "What could I do? Storm a guarded prison by myself to free them?"
  
• "I had enough problems of my own, like working all day to feed my kids and trying every day not to lose my job. I just didn't have the time to worry about those people."
  
• "Even if I took part in a protest, the government wouldn't listen. The government never listens."
  
• "I didn't want to believe the abuse was so bad, I told myself those were isolated incidents or exaggerations."
  
• "As a police officer I didn't like the way prisoners were treated, but they had to be kept off the streets. Besides, it wasn't me sentencing these people or guarding the prisons."
  
• "When they imprisoned my neighbor, I didn't think he'd be abused. I thought he'd serve his time comfortably."
  
• "I thought only rapists and murders got abused."

The point is, that's just what happens in situations like this. We distance ourselves from the injustice that is happening, we tell ourselves we're powerless to do anything or that it's not our place to intervene, we convince ourselves that "it's not that bad" or that "they deserve it on some level", and even when we take part in it and facilitate it we tell ourselves that we had no choice.

But if prisoners were suddenly "saved" by an powerful armed force who told us "This is very, very wrong" and we were brutally confronted by what goes on in prisons (like seeing actual footage of abuse with our own eyes), we'd be horrified... Just like the Germans when they were shown footage of these things.

The explanations to the Germans' behavior lie under our very noses: We are doing the exact same thing today and we just need to reflect on our own inaction to understand the inaction of the Germans.

With that said, please note that I am not criticizing people for not doing anything about abuse in prison. I think that this apathy is just normal human behavior when facing injustices that seem much bigger than our own individual selves.

And yes, there were exceptions - Germans who tried to protect Jews for instance, but these were exceptional actions. Those people did something amazing by breaking free of these apathetic mental processes and even risking their lives helping others, but I don't think it's fair to hold everyone to that standard and expect everybody to be exceptional like this. We're just human, we are flawed and weak. Those of us who have the fortitude to go out of their way to do the right thing are often lucky for having that ability.

Also no part of my post is meant to defend or justify the holocaust. I'm just trying to offer an explanation for Germans who turned a blind eye to it. If you want to know such things as why an SS soldier would personally torture a prisoner, or what drives a man to pull the lever that will release deadly gas in a room full of people, the explanation isn't in this post.

And by the way, if some people don't think that abuse in modern prisons is comparable enough to concentration camps, then think of Gitmo and ask yourself why you didn't do anything about that. Gitmo was in many ways a modern day concentration camp with torture and inhumane treatment of prisoners. Even if most prisoners there were terrorists, many were innocent and it could also be argued that many of the "terrorists" were only people trying to defend their country from what they perceived as an invading force who threatened their families (which morally is a bit different from terrorists who intentionally target and kill civilians, I think).

How many people actually did anything significant, at great personal cost, to put an end to Gitmo? I mean doing something beyond posting against it on Facebook and taking part in a few peaceful protests in the streets? And why didn't they do more? Answer that question and you'll have the answer why so many Germans acted like the Holocaust wasn't happening.

If anyone still doesn't think that modern prisons or Gitmo are a good comparison in any way, I'm not going to argue with you. My point isn't to convince anyone that Prisons/Gitmo are like Auschwitz. No, my point is that if you want to understand why most Germans didn't act against the Holocaust, just try finding examples of contemporary injustices and human rights abuses you aren't acting against (preferably incidences occurring close to you), and think of why you aren't acting (or doing more). I happen to think the comparison to prisons/Gitmo works for that purpose, and some people will agree with me and some won't. That's fine, if that's the case I'm just sorry I can't come up with a comparison that would better satisfy you and would help you understand the behavior of the Germans.