Save a thief from the gallows and he will cut your throat.
More French Proverbs:
The cask always smells of the herring.French
Don't run too far, you will have to return the same distance.
French
The tongue wounds more than a lance.
French
Truth is the club that knocks down and kills everybody.
French
A fool's tongue is long enough to cut his own throat.
French
In the fiddler's house every one is a dancer.
French