An old poacher makes the best gamekeeper

A theef of venysoun, that hath forlaft His likerousnesse [depravity] and al his olde craft, Kan kepe a forest best of any man.
[c 1390 Chaucer Physician's Tale l. 83]

Alwayes set a—to catch a—; and the greatest dear-stealers, make the best Parke-keepers.
[1695 T. Fuller Church Hist. Britain ix. iii.]

There is a saying that an old poacher makes the best gamekeeper, on the principle of setting a thief to catch a thief.
[1878 R. Jefferies Gamekeeper at Home ix.]

What the Church needed, possibly, was a good leavening of sinners in its ministry, on the principle that poachers make the best gamekeepers.
[1970 V. Canning Great Affair iii.]

‘You'll have heard the saying that a poacher makes the best gamekeeper?‥Well, Dodd's the reverse. He's a gamekeeper who would have made the best poacher.’
[1978 T. Sharpe Throwback i. 5]

Related to: guile; wrong-doers

Bibliography of major proverb collections and works cited from modern editions is available here.