Sonboly had undergone psychiatric treatment for depression, anxiety and post-traumatic stress disorder. In 2015, he had been treated for two months as an inpatient in a mental care facility. He was largely isolated from his peers and suffered years of bullying by classmates, which included physical abuse. A former classmate recalled, "We always him in school, and he always told us that he would kill us". He developed a hatred for those of a similar age and background as those who bullied him, mainly Turks and Albanians. He developed an "irrational worldview" in which he believed that the people he hated were infected with a virus and must be exterminated. On the day of the shooting, Sonboly saved a document on his computer in which he wrote "The bullying will be paid back today". During the shooting itself, he shouted that he had been bullied for seven years.
