Human languages are complicated, messy, and sometimes lacking reason. They evolve out of a practical need to communicate, and the become more complex as we gain a desire to express less practical ideas and concepts. A stone is simple, and so is a wheel. Wrap that wheel in rubber, give that rubber treads, and bolt on a hub cap, and suddenly the simple word "wheel" means so much more. This process isn't very difficult to understand when it comes to the material world. We create new things, and come up with new words to describe them. However, less tangible concepts are at the mercy of the people that use them.
We give words power!
A word is not harmful, but the connotation behind those words can be powerful. The meaning we give words are what gives them power, and what evokes the emotional response. If I say "fuck" after hammering my thumb, only some people may mind. However, if I tell a person to "fuck off", there's a much stronger connotation behind it. Why would fewer people mind in the first case, but more in the second? The word is powerful in that some in society view it as a dirty word. It's foul, and only because those people see it as such. In their minds it's a word used by the mean, the harsh, the criminals, and the unsightly. As we normalize, the extremes aren't so extreme anymore-- more "normal" people use the word. In the second example, the word usually holds a very negative connotation. It expresses a strong desire for someone to leave-- a hateful desire. A person can purposely decide that the word no longer holds such weight, but the intent behind it being spoken is still there. The very person who said it still recognizes the connotation, and still harbors those strong feelings.
The words that have the most power are often the messy words we use to convey our feelings-- the less tangible concepts.
Racial slurs are typically very offensive. More than any other category of words (that I can think of), racial slurs are invented out of extreme prejudice and hate. They embody every negative emotion a person could convey to another, and possibly represent the physical turmoil those hated people have suffered throughout history. These words represent some of the most irrational, mindless, hateful expressions we can possibly make to another human being. They are the very example of how we give words power. They are perhaps the very example of what is wrong with us as a race.
Yet these are still words, and like any other words we can strip them of their power.
We normalize, we strip away the power some of these hateful words have. Over time they lose power, and are eventually forgotten. While they are still around, one still needs to understand that they hold a connotation, and that those connotations are often very powerful.
So, why when someone angrily shouts the word nigger, they're reprimanded, but not while during casual conversation? The variables are many, but it is often the intent. The things people say when angry are often loaded with negative intent, and are used most often to harm the mental condition of another human being. In casual conversation, the word has simply been normalized to the point that there isn't any negative intent. Then again, perhaps the word is merely said in like-minded company.
It is my belief that people decide to be politically correct to spare people from the very power we give words. It is something employed out of compassion, but most often as a veil to hide a person's true intent and feelings. To be politically correct is often deceitful and dishonest; it's just another dirty tool.