lone gunman

(idiomatic) An individual person who acts on his or her own initiative, without partners, especially one who has sole responsibility for doing something questionable, confidential, or iniquitous.

Example: 2002, Jed Mercurio, Bodies, ISBN 9780224061971, p. 340:
  He continues, "So when someone blows the whistle, the hospital has to start looking for a scapegoat. The hospital looks for the lone gunman—the individual, acting alone, acting counter to his training, who, through a gross act of incompetence . . . ."
2008, John Grisham, The Appeal, ISBN 9780440244974, pp. 197-198:
  Trial lawyers, always a colorful and eclectic bunch. . . . Most were lone gunmen too eccentric to keep much of a staff.
2008, Lee Raffel, I Hate Conflict! Seven Steps to Resolving Differences with Anyone in Your Life, ISBN 9780071484893, p. 260:
  The Wilson family had been sorely conflicted because each member had gone off in his or her own direction, thinking that they were all lone gunmen.