Wiktionary
n. (context idiomatic English) 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.
Usage examples of "lone gunman".
Whoever had sent the Tigress, Grum, the chlorofiend, and the lone gunman after me had been trying to kill me.
Whoever had sent the Tigress, Grum, the chlorofiend and the lone gunman after me had been trying to kill me.
Until now I had thought of Robertson as a lone gunman, a mental and moral freak scheming toward his bloody moment in history, like so many of those infamous scum in his exquisitely maintained files.
Swat teams were at the site and had a lone gunman pinned down in a row house.
I'd say what we have here is a lone gunman who hides good on the ridge lines, or somebody playing two-faced.