One key quote crystallizes the moment that Elizabeth's prejudice against Darcy forms. After she overhears him saying she is tolerable-looking but not pretty enough to tempt him to dance, she is offended, as any young woman would be. Having heard this insult:

Elizabeth remained with no very cordial feelings toward him.

This is classic Austen understatement. "No very cordial feelings" means she dislikes him intensely and is angry at him.

When Elizabeth is helping to care for the sick Jane at Netherfield, she notices Darcy looking at her. As she has developed the firm idea that he hates her, she can only imagine he is looking at her in a way that is critical of her. She assumes that she displeases him. She thinks:

She liked him too little to care for his approbation.

In other words, she tells herself she doesn't need his approval since she hates him anyway.

When Darcy proposes marriage to her, clueless about how much he is insulting her with his arrogant assumption he is doing her a favor, she says:

Had not my feelings decided against you—had they been indifferent, or had they even been favourable, do you think that any consideration would tempt me to accept the man who has been the means of ruining, perhaps for ever, the happiness of a most beloved sister?

One of the sources of dislike is her deep anger over Darcy's role in encouraging Bingley to drop Jane, as she expresses above. However, as she notes, her "feelings decided against" him: in others words, she would dislike him even if he hadn't hurt her sister.

Elizabeth Bennett is not without her own fair share of pride, and Darcy's insensitive first marriage proposal really offended her.

In spite of her deeply-rooted dislike, she could not be insensible to the compliment of such a man's affection, and though her intentions did not vary for an instant, she was at first sorry for the pain he was to receive; till, roused to resentment by his subsequent language, she lost all compassion in anger." (Ch. 34)

Elizabeth, though incensed, still tries to be civil to Darcy, despite her feelings for the man, but at some point, enough is enough--she just could not listen to him abuse her family and low connections. 

In the quote above, Elizabeth was just analyzing her mix of feelings on refusing Darcy.  Of course, Darcy is utterly shocked that she would not consider his very eligible hand in marriage; he had entered the proposal fairly confident that she would say 'yes.'  The following quote really sums up the extent of Elizabeth's feelings on the subject. 

"From the very beginning— from the first moment, I may almost say— of my acquaintance with you, your manners, impressing me with the fullest belief of your arrogance, your conceit, and your selfish disdain of the feelings of others, were such as to form the groundwork of disapprobation on which succeeding events have built so immovable a dislike; and I had not known you a month before I felt that you were the last man in the world whom I could ever be prevailed on to marry." (Ch. 34)

Elizabeth, here, is extremely straight-forward with Darcy.  He had wanted to know her reasons for refusal, so she lets him have it.  Of all the sorts of responses Elizabeth might have given, Darcy was not expecting to hear her say this.  In short, the quote really comes down to a personal attack on his character: selfishness, conceit, arrogance.