Jean Eric Gassy was born on May 11, 1956, in Australia. He pursued a career in medicine, specializing in psychiatry. Gassy worked in various medical institutions, including St George Hospital in Sydney.

In 1994, concerns about his behavior and mental health led Dr. Margaret Tobin, then director of mental health for the Southern Sydney Area Health Service, to request an evaluation of Gassy's fitness to practice. He had been on extended sick leave and exhibited paranoid behavior, conflicts with staff, and inappropriate conduct. Despite differing opinions among psychiatrists regarding his condition, Gassy was eventually diagnosed with a delusional disorder. His refusal to comply with treatment recommendations and conditions led to his deregistration as a medical practitioner in 1997.

Following his deregistration, Gassy harbored resentment towards those he believed were responsible, particularly Dr. Tobin. He maintained that he did not have a delusional disorder and believed he was the victim of a conspiracy. Gassy's fixation on these beliefs persisted over the years, culminating in a meticulously planned act of violence.

On October 14, 2002, Dr. Margaret Tobin, then the director of mental health services in South Australia, returned from lunch to her office in the South Australia Health Department building in Hindmarsh Square, Adelaide. As she exited the elevator on the eighth floor, she was shot four times and died shortly after.

The investigation led to Jean Eric Gassy, who had driven from Sydney to Adelaide under an assumed name. Evidence included gunshot residue in his rental car, ownership of firearms matching the murder weapon, and a "hit list" of medical professionals he believed had wronged him.

Gassy was arrested in November 2002 and represented himself during his trial. In October 2004, he was convicted of Dr. Tobin's murder and sentenced to life imprisonment with a non-parole period of 34 years.

However, in May 2008, the High Court of Australia quashed his conviction, citing that the trial judge's directions to the jury were not sufficiently balanced, leading to a miscarriage of justice.

A retrial commenced in April 2009, and on May 6, 2009, Gassy was again found guilty of murder. This time, he was sentenced to life imprisonment with a reduced non-parole period of 30 years.

In August 2023, Gassy sought to have his sentence reduced, alleging that Dr. Tobin had attempted to infect him with HIV in 1994. The Court of Appeal dismissed his claims, stating that none of the proposed grounds had merit.