Elizabeth Bennet is making a comment in a general conversation involving Caroline Bingley, Darcy and herself, that Darcy suffers from no imperfections of character. She accuses him of feeling superior to everyone, literally she says to him that because of his feelings of superiority that he just hates everyone.
He openly feels contempt for others, who he sees as beneath him in character and does not try to hide his feeling of disgust at the inferiority of those he meets. Darcy accuses Elizabeth of misinterpreting people's intentions. Darcy's reply to Elizabeth's accusation follows:
"I cannot forget the follies and vices of others so soon as I ought, nor their offences against myself. My feelings are not puffed about with every attempt to move them. My temper would perhaps be called resentful. -- My good opinion once lost is lost for ever.'' (Austen)
Elizabeth counters with the fact that his inability to accept the faults of others is a character flaw.
That is a failing indeed!'' -- cried Elizabeth. Implacable resentment is a shade in a character. But you have chosen your fault well. -- I really cannot laugh at it; you are safe from me.'' (Austen)
Darcy's attitude toward the faults and mistakes of others really bothers Elizabeth, because he has perceived her own mother and sisters to be inappropriate. He does not suffer fools lightly, as the saying goes. Darcy takes the behavior of other people very seriously, as serious social infractions against the way people are expected to behave.
Darcy tells Elizabeth that his judgement on human nature comes from a belief that some people cannot be educated to behave appropriately.
"There is, I believe, in every disposition a tendency to some particular evil, a natural defect, which not even the best education can overcome.'' (Austen)
Elizabeth does not understand Darcy's prejudice against people who he believes behave in an inappropriate way, he has no tolerance for errors in social behavior. To Darcy behavior is very important. Darcy accuses Elizabeth of pridefully, willfully misunderstanding his perception of social behavior.
"In Chapter 11, Darcy says that every person has a defect in character that makes him blind to the goodness of others. Elizabeth answers that his defect is to hate everybody. Darcy countermands with “and yours is a willful ploy to misunderstand them.” Both show they have a defect, because of either pride or prejudice, and they need to surmount it."