Give enough rope and he'll hang himself
give (someone) enough rope
If you give someone the opportunity to do something wrong or detrimental to themselves, they will usually do it; one does not need to interfere to bring about someone's downfall. The full version is "Give one enough rope, and one will hang oneself." Don't interrupt him with questions. Just let him keep talking and he'll incriminate himself. Give him enough rope and see what happens.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Give someone enough rope and he'll hang himself
. Prov. If you give someone that you suspect of bad behavior the freedom to behave badly, eventually he or she will be caught and punished. Jill: I think Matilda's been stealing things out of my desk. Should I tell the boss? Jane: No; give her enough rope and she'll hang herself. One of these days she'll steal something important, the boss will find out for himself, and he'll fire her.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.