"Blomkvist thought it looked like a Colt .45 Magnum – the kind of weapon used to murder Olof Palme."

Olof Palme was something of a political hero of the left, detested by the right. Check out this tribute from YouTube. As has already been referenced, he was assassinated in 1986. He was out spoken against the U.S.A. and the Vietnam War. Larsson was himself an anti-Vietnam was protester; clearly he must have been one of Palme's supporters.