Much to much Psychology. Nothing is ever anyone's fault. It's always the underlying 'drives' that are to blame. Personal responsibility is completely missing. That their actions were their own choises, were sin, is never mentioned. 
 It felt that two much of the story was used setting up the background. The crime, investigation, and solution seemed tacked on, not really connected to the rest of the story. The solution especially. The setting, Petra, really has nothing to do with the story. 
 I didn't like this one, even though Poirot was the detective.