wolf in sheeps clothing
wolf in sheep's clothing {n. phr.}
A person who pretends to be good but really is bad.
Mrs. Martin trusted the lawyer until she realized that he was a wolf in sheep's clothing.
Mr. Black was fooled by the salesman's manners until he showed that he was really a wolf in sheep's clothing by selling Mr. Black a car that was falling apart.
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Source: A Dictionary of American Idioms