Is everyone in The Bad Seed crazy (Leroy, Rhoda, the mother and father)?

It has long been debated whether anti-social and sociopathic behavior is the result of nature or nurture. The nature side of the debate holds that some people are “born bad," arguing that some very young children behave in antisocial ways and that some families exhibit antisocial behavior across generations.
The proponents of the nurture side of the debate state that evidence indicates that antisocial behavior stems from a childhood rife with neglect, abuse, violence, deprivation, and poverty and that some or all of these properties combine to cause such distress that antisocial and psychotic behavior can result. They postulate that, since family members grow up in similar circumstances, it makes sense that the resulting personality disorders appear to run in the family.
A more modern approach combines nature and nurture, as it is recognized that there are several forms of mental illness that do have genetic components. However, a loving and therapeutic approach can moderate the genetic predisposition, while an antisocial environment can cause antisocial behavior.
So, as to whether the characters in The Bad Seed are “crazy," the very word indicates a lack of understanding of causation and motivation. Each character perceives and reacts to what they experience in different ways. The child undoubtedly has a severely antisocial personality and reacts with violence and a lack of empathy. The adults react outside of the boundaries of rational behavior, but actions based on emotions are not necessarily the product of mental illness.

William March's The Bad Seed appears to suggest that some people are born inherently evil ("nature over nurture"). So, one might think March portrays everyone in the story as crazy, evil, or sociopathic. But several major characters do not exhibit these traits.
Rhoda, the titular character, is shown to be naturally murderous—she appears to have inherited sociopathic tendencies from her grandmother. Her mother, Christine, somehow escaped being either a killer or psychotic, although some may interpret her suicide as a sign of mental instability. Leroy, the maintenance man, is shown as depraved but not necessarily insane.
Claude is one character who is certainly not insane. He is a shy boy and a victim of Rhoda's. Kenneth, Rhoda's father, is also depicted as perfectly stable, and he is unaware of Rhoda's nature and actions. Three members of the Fern family are also sane: Claudia, Octavia, and Burgess. A babysitter, teacher, and elderly neighbor are also shown to behave rationally and sanely. March's argument is that the crazed and evil instinctively recognize each other, while the good are easily fooled.