Guilherme Taucci Monteiro was born on July 5, 2001, in Mogi das Cruzes, São Paulo, Brazil. His early life was marked by challenges. He lived with his grandparents due to his mother's struggles with addiction. Guilherme was once a student at a local school but dropped out after facing bullying. This difficult experience impacted his mental health.

From a young age, Guilherme was interested in goth culture and showed an unsettling admiration for Nazism. He developed a close friendship with Luiz Henrique de Castro. They met as children and spent a lot of time together, often going to the mall or visiting a LAN house to play first-person shooter games.

In terms of work, Guilherme did various odd jobs to earn some money. He worked as a vendor at a hotdog stand, where he made around 600 reais (about $114). Guilherme's grandfather mentioned that he often went to the LAN house with Luiz and was not surprised when his grandson had money to spend on games or online purchases.

Guilherme's life took a darker turn after the death of his grandmother four months before a tragic event in which he would be involved. He started showing signs of clinical depression around this time. Despite his struggles, he held on to plans for his future, partially inspired by a promise from Luiz's father to help him find a job in maintenance services in São Paulo.

Guilherme Taucci Monteiro and Luiz Henrique de Castro committed a tragic attack on March 13, 2019, at the Professor Raul Brasil State School in Suzano, São Paulo, Brazil. Earlier that day, at around 9 a.m., they killed Taucci's uncle, Jorge Antônio Moraes, at a nearby car shop. This act was part of a planned series of events leading to the larger attack.

At about 9:40 a.m., the two entered the school wearing masks and combat boots. Taucci was the first to enter the building. He shot two staff members and several students. After this initial attack, he moved into the main patio in search of more victims. Meanwhile, Henrique entered shortly after, also armed and carrying multiple weapons. He approached the injured and struck them with a hatchet.

The attack resulted in the deaths of five students and two school staff members. A total of eleven others were injured during this incident. The police quickly responded, securing the building and later discovering various weapons, including a revolver and a crossbow, and they had to send in a bomb squad to investigate a potential explosive device, which was later confirmed to be fake.

Following the attack, Guilherme Taucci took his own life after killing Henrique, marking a tragic end to the day’s events. The actions of Taucci and Henrique were deemed as part of a long-planned attack influenced by past incidents of violence, leading to significant repercussions and discussions about school safety and violence in Brazil.