To what extent does the story explore the motive for the behavior of the wife and husband, the husband's mother, the servants, and the people who surround the suburb and the house?

None of the characters says anything overtly racist in the story, but racism is still there, simmering just below—and, occasionally, just above—the surface. The "wise old witch" warns her son and daughter-in-law "not to take anyone off the street." The housemaid and gardener are Black, yes, but they are "trusted" and "highly recommended," which contrasts with all of the others. The housemaid criticizes other members of her race in order to set herself apart. The couple comforts themselves that they are not racist because their sign has a silhouette of a person who could be either white or Black. All of these examples contribute to the subtle racism that the couple possesses. They think they are not racist because they aren't openly aggressive about it, but they are fundamentally shaped by a sense of their own white supremacy such that every comment, every thought, and every action in the story is about keeping themselves safe from the dangerous, rapacious, threatening Other.