It's not surprising that Elizabeth should have rejected Mr. Darcy's overtures. She's always found him so stern and unbending, a proud, aloof snob with a perpetually cold, icy demeanor. Even when he subsequently writes to her, explaining his unacceptable behavior, he merely compounds the offense, telling her bluntly that the Bennet family is vulgar, lacking both wealth and propriety, the things that really matter in this status-conscious society. Hence his attempt to break up the blossoming relationship between his good friend Bingley and Jane.
Nonetheless, Darcy partially redeems himself in Elizabeth's eyes by giving her the lowdown on Mr. Wickham. Like just about everyone else, Elizabeth has been thoroughly taken in by Wickham's good looks and charm. Until she reads Darcy's letter she never would have thought in a million years that he was such a gold-digging ne'er-do-well, a worthless rake who tried to elope with Darcy's sister for her money despite his being given a generous bequest by his adoptive father, Darcy's old man.
This information is crucial as it presents Darcy in an altogether different, more sympathetic light. Elizabeth still can't pretend to be thrilled at Darcy's interference in Bingley and Jane's romance, but at least she's starting to realize that, beneath that cold, forbidding exterior, there is a man with a sense of honor, decency, and integrity.