a wolf in sheep's clothing 披着羊皮的狼；伪君子；口蜜腹剑的人

【例句】
He is a wolf in sheep's clothing, outwardly kind but inwardly vicious!
他是个披着羊皮的狼，外貌仁慈，内心狠毒！

The whole truth came out at last that he was a wolf in sheep's clothing.
真相终于大白了，他原来是只披着羊皮的狼。