A resident of St. Petersburg, Kazantsev was allegedly an active member of the Right Sector, known by the alias Sever. On March 24, 2022, he was taken into custody after being found among Ukrainian prisoners of war. Following his detention, the NTV TV channel aired a video showing his confession. His mother claimed that the confession was coerced through torture, which led to the amputation of his toes. In court, Kazantsev denied any connection to the Right Sector. On November 28, 2023, Kazantsev was sentenced to 12 years in a strict-regime colony on charges of participating in an extremist organization, possessing weapons, and participating in an armed formation. On March 19, 2024, the appeal court reduced his sentence by one month.