Black and White
In many old western movies, there was a stereotype: the good guys wore white hats and the bad guys wore black hats. Now that not so many people wear hats, that's not quite so easy to arrange, but movies still often have two kinds of characters: good and bad. Some movie villains are so diabolically evil that they go beyond hurting others for selfish gain - they cause pain just for the sheer joy of it. Many good guys are virtually saints totally incapable of an unkind act. A few movies involve character studies, where all the characters have some strengths and some weaknesses, but these are often not as popular at the box office. We should remember that the extreme stereotypes often used in entertainment do not reflect real life.
Nevertheless, it isn't uncommon to hear people talk as if the world were divided into criminals and law-abiding people. But people are very complex. A person who has never had trouble with the law might be extremely selfish and nasty. Another person might be convicted of crimes repeatedly because of drug abuse or irresponsible companions, but still be sensitive to the feelings of others. Of course the extremes are still likely to exist in some cases - a few criminals probably do come close to the fictional stereotypes of evil, and some honest people are so wonderful that nobody can find a bad word to say about them. A wide variety of people exists, each of us with a complex set of strengths and weaknesses.
It is tempting to judge people by simplistic standards. Someone who is seriously overweight might be assumed to have no self control, someone who dresses poorly might be assumed to be poor or low class while a person who wears expensive clothes might be seen as a big-shot. Similarly someone who spells poorly or uses bad grammar might be assumed to be ignorant while a person who uses big words is judged to be smart or wise. We should always be careful about putting people into categories based on stereotypes and insufficient evidence.
When there is a controversy, people will often line up on one side or the other. When the people on each side tend to talk mainly with people of the same viewpoint, they constantly hear how good the arguments for their side are and how bad the opponent's arguments are. Those on each side begin to consider themselves morally superior and their opponents stupid or inferior. They begin to see things in black and white - their own side consists of good guys and the other side is bad guys. It can even get to the point where anyone who takes an intermediate position is considered bad, as exemplified by the slogan "If you're not part of the solution, you're part of the problem."
There is more extensive coverage of polarization (here)