We can see Atticus’ attitude toward the word “nigger” in how he talks to Scout early in chapter 9. Scout has been hearing hateful talk at school about the fact that her father is defending a black man. At home, she asks him, “Do you defend niggers, Atticus?” Atticus tells her not to use the word “nigger” because it’s “common.”
What he means here is that the word is used by people who don’t try to rise above the hateful prejudices that they have grown up with. Inheriting such attitudes is what the masses (the “common folk”) typically do. Notice that Atticus is very circumspect with how he says it, however. He doesn’t do any name-calling or hate-mongering of his own. It’s his style to stay above the fray.
As for defending Tom Robinson, he explains this to Scout in terms of personal integrity. After she asks why he’s defending him, he says,
“For a number of reasons. The main one is, if I didn’t I couldn’t hold up my head in town, I couldn’t represent this county in the legislature, I couldn’t even tell you or Jem not to do something again.”
For Atticus, personal belief and morality must always be reflected in personal actions.