Tommy Zethraeus was born on May 28, 1969, in Botkyrka, Sweden. Little is publicly documented about his early life. Prior to the 1994 incident, Zethraeus had a criminal record, including a 1990 arrest. A forensic psychiatric evaluation later revealed damage to his frontal lobe, which may have impaired his ability to understand the consequences of his actions.

In the early hours of December 4, 1994, Zethraeus and two friends were denied entry to the Sturecompagniet nightclub in Stockholm. After an altercation with the doormen, they left but returned around 5:00 a.m. Zethraeus, armed with a Norwegian AG-3 battle rifle, opened fire outside the club, killing four people: doorman Joakim Jonsson (22), and patrons Katinka Genberg (21), Daniella Josberg (22), and Kristina Oséen (21). Over twenty others were injured in the attack.

Following a three-day manhunt, Zethraeus and his accomplices were arrested on December 7, 1994. In September 1995, he was convicted of four counts of murder and sentenced to life imprisonment. His accomplices received shorter sentences.

Zethraeus applied multiple times to have his life sentence commuted. In 2014, a court granted his application, setting a conditional parole date in 2016. However, this decision was appealed and later overturned. He was eventually released in December 2020 after serving 26 years.

In 2022, Zethraeus was re-arrested and sentenced to 18 months in prison for aggravated drug trafficking, involving assistance in importing significant quantities of cocaine. His parole was revoked, and he is currently incarcerated, with eligibility for release after serving a minimum of three years.