an old poacher makes the best gamekeeper
People who have a lot of experience in a criminal activity are the ones best suited to trying to protect against that activity later in life. A: "I heard that computer company is planning on hiring renowned hackers to run their security infrastructure." B: "Well, old poachers make the best gamekeepers." A: "The new sheriff used to be a notorious thief?" B: "Hey, you know what they say—an old poacher makes the best gamekeeper."
old poacher makes the best gamekeeper
Prov. The best person to guard something is someone who knows all about how to steal it, so he or she can anticipate what thieves might do. We should hire the computer hacker to design computer security systems. An old poacher makes the best gamekeeper.