poacher turned gamekeeper

Etymology
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 "

Noun

 * 1)  A person now charged with protecting what they formerly preyed upon.