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In “The Devil’s Castle: Nazi Eugenics, Euthanasia, and How Psychiatry’s Troubled History Reverberates Today,” Susanne Paola Antonetta examines T4 in light of her own experience with bipolar disorder since childhood in the 1960s—and with treatments, such as shock therapy, that were sometimes futile or worse.

“I researched that euthanasia story with the urgency of someone who could have been a victim,” she writes.

But Ms. Antonetta reminds us that several T4 perpetrators escaped justice and went on to practice medicine in postwar Germany.

That story includes lobotomy, electroshock and overmedication.

German psychiatric researchers of the early 20th century sought the causes of schizophrenia and other disorders in brain biology, an intellectual current that influenced the doctors who later carried out T4, as Ms. Antonetta writes.