Complicity — Iain M. Banks

Complicity n. 1. the fact of being an accomplice, esp. in a criminal act
Local journalist Cameron Colley writes articles that are idealistic, from the viewpoint of the underdog. A twisted serial killer seems to have the same Mo -- he commits brutal murders on behalf of the underdog. As the two stories begin to merge, Cameron finds himself inextricably and inexplicably implicated by the killer.
When the arms dealer whom Cameron plans to expose is found literally "disarmed" before Cameron can even put pen to paper and the brewery chief, loathed by Cameron, who sold out at the expense of his workers finds himself permanently unemployable, the police become convinced of Cameron's guilt, as do half his friends and colleagues, forcing Cameron to employ all his investigative skills to find the real killer and his motive.