I think the "N" word is a very derogatory and disrespectful. It should never be used under any circumstances. There are many other disrespectful words that may offend certain individuals but when you use the "N" word, it not only insults that person but is offensive to an entire race. When it is used in the movies we watch or the songs we listen to one would think it is ok to say. But it is not.
I believe Mark Twain used the word so often because of the period it was written in (1850's). The "N" word was commonly used when talking about or to African Americans. I also think that African Americans were accustomed to being referred to as the "N" word.
The "N" word should not be used amongst friends as a way of joking around or be said as a means to offend an African American. Used in either of the contexts is wrong and the "N" word is offensive regardless of what race it is said