set a thief to catch a thief
To employ a criminal or thief in order to understand, anticipate, and ultimately apprehend another criminal or thief. Honestly, who better to lead an investigation into tax fraud than someone who was convicted of just that? Set a thief to catch a thief, I say. We've actually begun employing prominent members of the hacking community to improve our cyber security, setting thieves to catch thieves, as it were.
Set a thief to catch a thief.
Prov. The best person to catch a thief is another thief, because he or she knows how thieves think. The government set a thief to catch a thief, hiring a stockbroker convicted of fraudulent practices to entrap the stockbroker they were investigating for fraud.