Chapter 21 is all about Anne and Mrs. Smith,who holds up quite a mirror to Mr. Elliott's true nature. After being convinced that

Anne has no intention of marrying him,Mrs. Smith subtly gives her
real opinion of him:

"I beg your pardon, my dear Miss Elliot," she cried, in her natural tone of cordiality, "I beg your pardon for the short answers I have been giving you, but I have been uncertain what I ought to do. I have been doubting and considering as to what I ought to tell you. There were manythings to be taken into the account. One hates to be officious, to be giving bad impressions, making mischief. Even the smooth surface of family-union seems worth preserving, though there may be nothing durable beneath. However, I have determined; I think I am right; I think you ought to be made acquainted with Mr. Elliot's real character. Though I fully believe that, at present, you have not the smallest intention of accepting him, there is no saying what may happen. You might, some time or other, be differently affected towards him. Hear the truth, therefore, now, while you are unprejudiced. Mr. Elliot is a man without heart or conscience; a designing, wary, cold-blooded being, who thinks only of himself; who, for his own interest or ease, would be guilty of any cruelty, or any treachery, that could be perpetrated without risk of his general character. He has no feeling for others. Those whom he has been the chief cause of leading into ruin, he can neglect and desert without the smallest compunction. He is totally beyond the reach of any sentiment of justice or compassion. Oh! he is black at heart; hollow and black!"

Whew-holding back quite abit there,we were! After going into his less than honorable intentions of grabbing on to his inheritance before Sir W remarries(which is a possibility but not,IMO,to Mrs. Clay, despite her potently preening prescence)thru Anne,she naturally asks her friend why she was encouraging the match at first,to which Mrs. Smith basically says"Well,it seemed like a sure thing that no one could talk you out of so I thought perhaps
it would be better to stay friends so that Mr. Elliott might be more inclined to help me out with this property to be reclaimed".