How do we explain Toshiko's strange and obsessive behavior?

Toshiko's strange and obsessive behavior is a product of her fears and insecurities. Toshiko is appalled at the crassness of modern Japan and fearful of what the future holds for a society mired in moral degradation and social decay.
To Toshiko, delicately reared and socially privileged modern Japan is the antithesis of true progress. Modernization has made her country repugnant, devoid of refinement and civility. Toshiko is as appalled by her husband's insensitivity as she is by the economic disparity in modern Japan. She muses that her life with her husband is "too easy, too painless." 
Toshiko becomes obsessed with the fate of the nurse's illegitimate baby, fearing that the baby will grow up to live a dissolute life. After all, even "if that baby should grow up in ignorance of the secret of his birth, he can never become a respectable citizen." More than anything, Toshiko fears the division of Japanese society into two distinct, mutually exclusive classes. In such a society, the rich and the poor will always be at odds.
Toshiko's fears inflame her thoughts, and she becomes paranoid. She imagines a horrific future, where the grown, illegitimate offspring of the nurse savagely stabs her (Toshiko's) son to death. Toshiko's strange and obsessive behavior is inspired by her fears and further exacerbated by the free exercise of her fevered imagination.