What is the significance of Mary Dempster's death for Dunstan?

Dunstan sees Mrs. Dempster as saintly, and throughout the novel he feels guilt of her behalf. It is somewhat ironic that the woman who provoked such feelings of guilt would also be the person to give up her life, but was it suicide? Dunstan's responsibility for Mrs. Dempster prior to her death an undesirable entity, and her actions are embarrassing to Dunstan. The finality of her death brings him a sense of relief.