The N word is a powerful slur that takes one out of their racial identity and has a connotation that is detrimental even without its meaning.
Step-by-step explanation:
Scout is moved to anger by the very mention of the N word and the cuss that he sees laced against himself is an insult he does not fully understand.
But, it is the tone with which the insult of N-lover is being conveyed in the passage and the connotation of betrayal of one's own race while demonizing other carries, the person who hears it is wont to display some anger even without understanding exactly what the word means, or what connotation it has to the scenario in the United states at the time of writing of the book.