lone nut
n. The alleged perpetrator of a large-scale or otherwise historic crime, usually an assassination (e.g., Lee Harvey Oswald, Sirhan Sirhan, Mark David Chapman, John Hinckley Jr.) Often suspected of being a mere patsy for a much larger and more sinister force.
"What do you mean, the Kennedy assassination was perpetrated by the CIA, the Mob, extraterrestrials, Johnny Carson and Mensa? Everyone knows it was a lone nut!"