The serial killer in the film The Snowman is Mathias Lund-Helgesen.
Throughout the investigation, detective Harry Hole and his team, including Katrine Bratt, consider several potential suspects for the horrific killings. Characters such as Arve Støp and Idar Vetelsen are among those initially suspected, leading both the protagonists and the audience down various investigative paths. However, it is ultimately revealed that Mathias Lund-Helgesen, who is present in the story from an early stage, is the true perpetrator behind the gruesome crimes, his identity as the killer being uncovered towards the end of the film.
