Ignorance
If there is one thing that all human beings share, it is our nearly entirely unmitigated ignorance of the world around us. If you took the knowledge of one person and divided it by everything a human being could possibly learn, the fraction would be effectively zero.
So it should not surprise us to learn that we have evolved to make decisions based on very limited evidence. And that we are often far more confident in those decisions that we have any right to be. (I think that everyone should read Thinking Fast and Slow by Daniel Kahneman. Social science has come a long way in showing how we make decisions. It's not super encouraging.) However, acting on incomplete information is essential to survival. When a large predator is chasing you down, you don't have time to do a lot of thinking. And just because we are no longer often chased by large predators, it does not mean that the shortcuts we take to make decisions have changed.
I believe that almost all the horrible things people do to each other are due to ignorance. I am not trying to make excuses for people who do horrible things. (Though I do admit to being purposely vague as everyone's definition of "horrible" will be slightly different.) But people do things for reasons. And that reason is almost never simply because they are evil.
People do not do things merely to illustrate their character traits. Suppose you observe a man cut you off in traffic. Many people's natural reaction will be to attribute this action to the man's character. "What an asshole." Yet if you were to somehow track this man down and ask him why he cut you off, he is probably not going to say, "I'm an asshole." He might not have seen you. He may have been driving his pregnant wife to the hospital. Or he might shrug it off (in which case, yeah, he's an asshole). Yet in any of these three scenarios, your reaction to being cut off probably would have been different had this information been available to you at the time.
Intentions and reasons matter to us. So while it may be difficult for some of us, when we see someone do something worse than just cutting someone off in traffic, we still need to ask why they are doing it. They might be ignorant. They might believe that their needs are more important at the time because of the circumstances. Or they just might be hateful assholes. Each of these scenarios has a different solution - education, compromise, and resistance.
We also have to be aware of our ignorance, especially our ignorance of others' ignorance. Sorry if that seems a little circular, but it's very easy to assume that other people know what we know, especially when it seems so obvious to us. When a person does something that you think is awful, you might ask yourself, "How can they not know that is awful?" My answer is, "Quite easily, unfortunately. Human beings really are that ignorant. Just as you are ignorant of the answer of your own question."
I do not want that response to be seen as too harsh. There should be no shame or stigma in being ignorant - as long as you are willing to admit your ignorance and learn from it. After all, I am as ignorant as anyone else. If, and it is a very big if, I have any more insight into this area than other people, it comes at the expense of my knowledge in other areas. I spend way too much time thinking about this shit. And I am ignorant of what this will cost me in the future.
Time and energy are limited resources. People will concentrate on what is important in their own lives. We should not be surprised when they are ignorant of our problems. We are most likely ignorant of theirs. It is a situation that can only be resolved through communication and - dare I say - empathy.