After Willoughby snubs Marianne at the party, these are Elinor's thoughts:

Her indignation would have been still stronger than it was, had she not witnessed that embarrassment which seemed to speak a consciousness of his own misconduct, and prevented her from believing him so unprincipled as to have been sporting with the affections of her sister from the first, without any design that would bear investigation.

Elinor is obviously still taken with the Willoughby they knew (or thought they knew) in Devonshire. And because he acted embarrassed to see them, she decides that he isn't totally unprincipled -- she seems to be giving him the benefit of the doubt, as much as possible.

Then, in the next paragraph, she thinks again about Edward:

Her own situation gained in the comparison; for while she could esteem Edward as much as ever, however they might be divided in future, her mind might be always supported. But every circumstance that could embitter such an evil seemed uniting to heighten the misery of Marianne in a final separation from Willoughby -- in an immediate and irreconcileable rupture with him.