Mrs. Smith had been carried away from her first direction,
Her “first direction” was a severe attack on the character of Mr. Elliot (“a man without heart or conscience; a designing, wary, cold-blooded being, who thinks only of himself; whom for his own interest or ease, would be guilty of any cruelty, or any treachery, that could be perpetrated without risk”) but she has been diverted into a discussion of Mrs. Clay’s design of marrying Sir Walter and Mr. Elliot’s efforts to prevent it.
and Anne had forgotten, in the interest of her own family concerns, how much had been originally implied against him;
Naturally Anne is interested in these revelations about Mr. Elliot’s relations with her family, but Mrs. Smith’s original attack on him was so strong that it implied more faults than she has revealed thus far.
but her attention was now called to the explanation of those first hints, and she listened to a recital
Mrs. Smith’s ‘recital’ is the next five paragraphs of the novel.
which, if it did not perfectly justify the unqualified bitterness of Mrs. Smith,
The narrator says that there is a difference between the level of bitterness that would be justified by the infamy of Mr. Elliot’s behaviour as described in this recital, and the level of bitterness displayed by Mrs. Smith. This points to either a flaw in Mrs. Smith’s character, or to the existence of something that Mrs. Smith knows but has not told Anne.
This level of discrimination is quite unrealistic—no-one can say to a hair exactly how much bitterness is justified by what level of infamy—but it reminds us that the narrator of Persuasion is paying close attention to the virtues and vices of even the sympathetic characters, and hence that so should we.