The term “bad seed” means a genetically evil person, someone who is corrupt by nature. This idea of preexisting morality is one that recurs throughout Christie’s works. When several characters have a motive to kill, the quality that distinguishes Christie’s murderer(s) is a moral compass that was defective from birth. Her writing tends to divide characters firmly into the categories of good and evil, with the evil characters (often murderers) being irredeemably and thoroughly bad.

After learning of her grandfather’s murder, Sophia worries that there is wickedness lurking somewhere in her bloodline. She’s correct; the bad seed of Crooked House’s Leonides family is 12-year-old Josephine, who turns out to be a double murderess. Though Christie portrays her as innately wicked, Josephine is a child who was ostracized by her parents due to her unattractive looks and strange personality. Her upbringing at Three Gables raises questions about whether her crimes are a result of nature, nurture, or a combination of the two.

When asked what makes a murderer, Arthur responds: “the brake that operates within most of us doesn’t operate with them” (95). Charles soon discovers that every person living at Three Gables had a potential motive to murder Aristide. Each family member has their own kind of ruthlessness, and each might have had a bone to pick with their late patriarch.