Lone wolf

They knew what it meant; even Joan had heard the cry of the lone wolf hunting in the lean time of winter, and of all things sad, all things lonely, all things demoniacal, the howl of a wolf stands alone.

Source: wiktionary

I am a lone wolf, a solitary man, wandering through a world in which I have no part. I am wifeless—childless—who is it speaks of the childless as the dead twigs on the tree of life?

Source: wiktionary

You'll never mix in. You'll be a lone wolf. I seen that right off. Wal, if a man can stand the loneliness, an' if he's quick on the draw, mebbe lone-wolfin' it is the best.

Source: wiktionary

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.

Source: wiktionary