] However, W. stated clearly he strongly dislike CB, with the result that Marianne changed her sentiment from feeling sorry for CB to also disliking CB.

Marianne and Willoughby each have their own reason for criticizing the Colonel in front of the other. As Barbara wrote in a previous post Willoughby saw Brandon as competition for Marianne. Marianne's reason is also linked to the fact that a match between Marianne and CB has already been openly talked of by their neighbors. So she may have wanted, by openly criticizing CB, to avoid Willoughby believing that there is a chance she may be contemplating that possibility and perhaps draw away from her as a result.