A wolf in sheep's clothing means someone who hides malicious intent under the guise of kindliness.
Example of use: "Don't trust the salespeople at the store; they are all wolves in sheep's clothing!"
The cautionary advice that one shouldn't necessarily trust someone since he or she is pretending to be something he/she is not, has been with us for many centuries. One form of it is Aesop's Fable in which a wolf who wanted to fool the shepherd into believing that he was a lamb, put on a sheepskin. When the shepherd locked the gate at night, the wolf thought it was safe to devour the sheep, however the shepherd returned at night to the flock to gather meat, and the wolf was slaughtered as one of the sheep.