Eve Tushnet on Muriel Spark’s The Girls of Slender Means (recommended):
Agatha Christie apparently liked Muriel Spark a lot, and one similarity I noticed–which goes along with the novel’s arch, judicious tone–is that both novelists paint human nature in shades of folly and wickedness. Those old-fashioned words (a Christie character in The Pale Horse explicitly points out how nobody calls things “wicked” anymore) have found no adequate modern replacement. Folly, in particular, is a category we have a hard time naming. Christie generally portrays even her characters who do great and lasting harm–the instigator/victim in The Mirror Crack’d, for example–as extremes of a trajectory the best among us follow now and then. Folly can destroy a life; folly is an inevitable tint in every human action. Folly is ridiculous and deadly, and normal.
Further reading: Tushnet on a rather different book, Florence King’s When Sisterhood Was in Flower (recommended).