It's in ch.16 that Elizabeth definitely changes her mind (for the worse) about Darcy, when she decides to believe Wickham's story:
"I had not thought Mr. Darcy so bad as this -- though I have never liked him, I had not thought so very ill of him. -- I had supposed him to be despising his fellow-creatures in general, but did not suspect him of descending to such malicious revenge, such injustice, such inhumanity as this!" (my emphasis - L.)
Up to this point, I think she had formed a fairly accurate idea of Darcy's character and outlook, but we'll have to see if her new lower opinion of him holds true! ;-)