Cérémonie, La (1995) review

Sophie (Sandrine Bonnaire) accepts a job to work as a maid for affluent Catherine Lelièvre (Jacqueline Bisset), her husband Georges (Jean-Pierre Cassel) and their children Gilles (Valentin Merlet) and Melinda (Virginie Ledoyen), but when Sophie becomes friends with the deranged postmistress Jeanne (Isabelle Huppert), who has a grudge against Georges, tensions escalate.  Chabrol based this on real-life sisters Christine and Léa Papin, who were housekeepers that murdered their employer's wife and daughter, except he made some questionable (and alienating) tweaks: both Sophie and Jeanne have reprehensible pasts (the former possibly killed her own father, the latter her four-year-old child) and behave belligerently, while the Lelièvres are consistently kind to their new hire and seem like a nice group of people (and don't deserve their fate).  So what's the explanation for the brutal murders at the conclusion?  Could it be a result of what Durkheim called anomie?  Are they sociopaths?  Or is it just that bad companions can take you to dark places?