Set a thief to catch a thief.

Related:

• Send a thief to catch a
  thief...
• There's no thief like a bad book. --
  Italian...
• It is a beggar's pride that he is not a thief.
  Japanese...
• He who holds the ladder is as bad as the thief.
  German...
• He who steals once is always a thief.
  Spanish...
• It is not the thief who is hanged, but one who was caught stealing.
  Czech...
• A thief believes that everybody
  steals...
• Constipation is the thief
  of time...
• Destroy the thief,
  my pets...