lone wolf

noun3

1. •
   A wolf that is not part of a pack.
2. •
   (figurative) A person who avoids the company of others; an independent or solitary person; a loner.

   • Don't be a lone wolf. Publish your results; take the world—take the nation at least—into your confidence. — 1897
3. •
   (figurative) A criminal who acts alone, not as part of a group.

   • [Stephen] Paddock's motives remain unclear. Las Vegas Sheriff Joe Lombardo described the shooting as a "lone wolf" attack. "We have no idea what his belief system was," he said. — 2017