Monsters aren't monsters everyday and monsters aren't monsters to everybody. In fact most monsters are pretty decent human beings (to a pretty restricted set of people that is).
During the course of History multiple civilizations have come and gone, many conflicts have been waged, many horrendous acts have been committed. We like to think of this horrendous acts as fruits of madmen, as fruits of mad times, and as fruit of others. But in fact those horrendous acts were committed by quite normal people that just happened to see different people as others.
During the course of History multiple civilizations have come and gone and for the most part of them they referred to themselves with a denomination that could be roughly translated as "People" and referred to other people with words that could be roughly translated as "Others", "Things", "Creatures" making quite clear that they saw themselves as being somewhat superior. This goes out globally and in time to.
When people look at others as being something akin to other animals, its very likely that they treat those others as being animals: very little respect, very little dignity, very little empathy. Mistreatment follows, exploitation follows, and if necessary murder follows.
Let's take for instance Stalin. One of the most brutal human beings to ever walk in this planet. By most accounts we was mostly an ok guy before his first wife died, even though a little bit violent prone, but after that he said that "with her died any human feeling in him". By most accounts he truly loved that woman and he is also quoted as saying that other than his mother, Ekaterina Svanidze was the only other person he truly loved.
With almost everybody else around him Stalin was a callous monster. Still he seemed to enjoy being around some children and a few times he was able to share what seemed to be something very similar to human warmth.
But most of the time, and to most of the people, Stalin always saw himself as being superior. Stalin saw himself as more apt and more willing. Others were just commodities, insignificant peons that were used and discarded as it fitted his whim.
The real problem isn't this despicable trend in Stalin's personality. The real problem was that he had power. He had power to exploit and eliminate those he deemed unfit.
All of us share this trait, I'd guess, and most of us would be aware of the constant double standards that we use in our lives. First it's us, than are those that are close to us (there are several levels of proximity) and then there are the others. People that are different in some way, and are as seen as not being "real" people, but backward people at best.
Everyone of us has dealt with double standards at one time or another on our lives, but it is hard to admit that we use double standards ourselves. It is hard to admit that we excuse ourselves (or ones that are close to us) from things that we would immediately point our finger to if done by the other. This is called moral disengagement. It allows one to disable the mechanism of self condemnation in order to not apply to ourselves (or ones that are close to us) ethical standards that we'd normally use.
The so called monsters do have morals (Try to kill a monster's loved one. Try to steal a monster. Try to rape a monster's loved one), even if only applied to a very strict set of people around him, even if applied with high levels of moral disengagement.
"Monsters" are monsters because they have power in their hands in opportunity in their hands. It isn't just about a set of conflicting values, that the "monster" may aware of or not, it isn't just about dehumanizing the ones that aren't near to you.
Both of these three factor have to happen in order for a "potential monster" to become a "real monster".
As soon as people all over the place realize that people all over the place are people all over the place, "monsters" will become rarer and rarer instances in our joint History. Because it is a joint History.
Why do I care?
How couldn't I care?