poacher turned gamekeeper

The figurative sense is by self-evident metaphor from the literal sense. The thought of poachers turned gamekeepers goes back centuries; for example, William Tinsley said in his memoirs that "There is an old saying that the most expert poachers make the best gamekeepers. I do not think the saying is true as a rule […] "

• "Ah! poacher turned gamekeeper! That be settin' thief to catch thief!"
• "When President Roosevelt appointed Kennedy to the Securities Exchange Commission, he was acting on the "poacher turned gamekeeper" principle."

• wolf guarding the sheep