Geza de Kaplany was born on June 27, 1926, in Mako, Hungary, into a wealthy family. His early life was marred by abuse; he lost sight in one eye due to a beating from his father, who died in 1938. De Kaplany pursued medicine at the University of Szeged, graduating with honors in 1951. He began practicing as a cardiologist in Budapest but fled Hungary following the failed 1956 revolution. After brief stays in England and Denmark, he moved to the United States, retraining as an anesthesiologist due to non-recognition of his Hungarian medical degree. He interned at Milwaukee Hospital (1957–1958), attended Harvard, taught at Yale, and eventually settled in San Jose, California, working at San Jose Hospital.

In June 1962, de Kaplany met 25-year-old Hajna Piller, a fellow Hungarian, former fashion model, and beauty queen. They married in August 1962. Shortly after, de Kaplany became convinced of her infidelity. On August 28, 1962, he tied Hajna to their bed, played loud music to mask her screams, and inflicted numerous cuts on her body with a scalpel. He then poured a mixture of hydrochloric, sulfuric, and nitric acids into the wounds, causing severe burns over much of her body. After three hours, he called the police, claiming he intended only to disfigure her, not kill her. Hajna survived for 33 days before succumbing to her injuries on September 30, 1962.

De Kaplany's trial began on January 9, 1963. He pleaded not guilty by reason of insanity, with his defense citing multiple personality disorder. Despite conflicting psychiatric testimonies, he was found legally sane and convicted of first-degree murder on March 1, 1963. He was sentenced to life imprisonment on March 15, 1963.

In a controversial decision, de Kaplany was paroled in 1975 to serve as a medical missionary in Taiwan. He later moved to Germany, becoming a citizen in 2000, which prevented extradition. As of 2002, he was living in Bad Zwischenahn, Germany.