His web history showed that Anton was most likely a xenophobe and that his motives for the murders was racism. However, he did not show any displays of mental illness but have had suicidal thoughts before the massacre. In a letter he left, he said that he wanted to be killed by a cop.
According to Aftonbladet, Pettersson had visited right-wing extremist groups at social media sites supporting Adolf Hitler and Nazi Germany. He had also joined a group on Facebook that wants to stop immigration to Sweden. The Swedish Security Service were called in to investigate these findings. Pettersson had no criminal record and was not a member of any political organisation, but had supported a petition by the Sweden Democrats to initiate a referendum on immigration. He left a handwritten note at his home in which he declared that something had to be done about immigration, and that he did not expect to survive his spree.
Pettersson graduated from Lichron Teknikgymnasium with a diploma in technology. He lived in an apartment in the neighbourhood of Stavre, but chose to attack the Kronan School in Kronogården, where there are more immigrants; police cited this as more evidence towards his motive. Former classmates described the perpetrator as a lonely person who "lived in his own world" and always dressed in black clothes influenced either by the emo or rock scene.
Bjørn Ihler, a survivor of Norwegian far-right extremist Anders Behring Breivik's terror attacks in 2011, wrote in The Guardian that in 2013, Pettersson had "liked" a YouTube video of former Ku Klux Klan leader Johnny Lee Clary testifying how a positive experience with a black man had caused him to disavow his previously-held beliefs.
He is known as the "Darth Vader" killer because he wore a mask that is similar to Darth Vader and said the character's line "I am your father".