Sergei Gordeyev was born on October 4, 1998, in Moscow, Russia. He was a 10th-grade student at School No. 263 in the Otradnoye District of Moscow. Known as an excellent student, Gordeyev was academically successful and participated in various school competitions. He aspired to graduate with honors and planned to attend the Moscow Engineering Physics Institute. Outside academics, he was physically active, attending sambo classes, and was a member of the "Vystrel" shooting club, where he learned to handle firearms under his father's guidance. His father, Viktor Gordeyev, was employed at a research institute affiliated with law enforcement agencies. Sergei's family also included his mother, Irina, an accountant, and a younger brother, Ivan.

Despite his achievements, Gordeyev exhibited signs of social withdrawal and had limited interactions with peers. He was fascinated by philosophical concepts, particularly solipsism—the idea that only one's mind is sure to exist. He believed that life was an illusion and that he was the only real being, viewing others as mere figments of his imagination

On February 3, 2014, around 11:40 a.m., Sergei Gordeyev entered School No. 263 armed with his father's legally registered firearms: a .22 caliber Browning SA-22 semi-automatic rifle and a 7.62×51mm NATO Tikka T3 bolt-action rifle. He also carried a dagger. Gordeyev threatened the school's security guard, who managed to trigger the alarm before following him to a classroom.

Gordeyev proceeded to his geography class, where he fatally shot teacher Andrei Kirillov in front of approximately 20 students. He then held his classmates hostage, discussing his philosophical beliefs and expressing a desire to experience death firsthand. When police officers responded to the alarm, Gordeyev opened fire, killing Warrant Officer Sergei Bushuyev and seriously wounding Senior Sergeant Vladimir Krohin.

Negotiations ensued, during which Gordeyev's father was brought to the scene. Wearing a bulletproof vest, Viktor Gordeyev entered the classroom and persuaded his son to release the hostages and surrender. By 1:05 p.m., Sergei Gordeyev was taken into custody without further incident.

Following his arrest, Gordeyev underwent psychiatric evaluation and was diagnosed with paranoid schizophrenia. On March 3, 2015, the Butyrsky District Court of Moscow ruled that he was not criminally responsible due to his mental illness and ordered compulsory psychiatric treatment. This decision was upheld by higher courts despite appeals from the victims' families.