It's interesting that you mention the "Miss McGrath" character, because apart from Frances' suspicions, the story never really tells you that there was anything between her and Paul. So if you consider adultery a sin, she might have been as innocent of that as of murder. I always notice that as she's walking away in that scene, she observes "custom" by not stepping directly on the front of a grave, sidestepping it instead. I always wonder whether that was planned, or just reflex on the actress' part.