Give him enough rope and he'll hang himself

I think this one has been used so much in so many different eras that we can lodge it in the same category of “if you lie down with dogs, you wake up with fleas” and “give him enough rope and he’ll hang himself.”

"Well, 'give him enough rope and he'll hang himself,' as they say," rejoined Mr. Munster—"that is, if your ideas about him are true."