In this brief chapter, the doctor continues in his reasoning with Villefort, saying that it must be the case that someone in the house has poisoned the Saint-Merans and Barrois, trying, in the latter case, to poison and finally kill Noirtier. The doctor says that the only logical killer is Valentine—that she apparently did not wish to be married, and was disinherited for a time by Noirtier, and so must have been protecting whatever money would be coming her way from both parties. But while Villefort is momentarily swayed by this, he concludes that Valentine cannot be responsible for these murders, for she is too pure a spirit. The doctor leaves, saying he cannot work in that house anymore, as it is a house of death.
A powerful instance of dramatic irony. While the reader may have guessed that Mme de Villefort is the culprit, the doctor seems to understand Valentine as the only logical answer. It is not clear why Valentine, who till this point has exhibited not a single negative emotion, could be a more plausible killer than Mme de Villefort, who seems devoted to her son’s happiness above all else (including Valentine’s wellbeing), but this is only to say that both the doctor and Villefort are blind to the events that are unfolding right in front of their eyes.
The servants, too, begin to leave, and Villefort wonders what will become of the family. He notes that Valentine is desperately sad at what has taken place, thus confirming that she was not in fact responsible for the death. But at the very close of the chapter, Villefort sees a “thin smile” curl across his wife’s lips, and he begins to wonder if it is perhaps Heloise who has been orchestrating the poisonings that are ruining his household and threatening its inhabitants.
Finally, Villefort seems to recognize that his wife, and not his daughter, is a more plausible culprit for these crimes. What remains to be seen is how Villefort will deal with the situation. For as he has already demonstrated, Villefort is most devoted to his own reputation, and wishes to defend that first before any other considerations arise, including the wellbeing of those around him.