面善心狠miàn shàn xīn hěn

a kind face but a ruthless heart; look kind but be cruel-hearted;innocent in appearance but a wolf at heart; bear the semblance of an angel but have the heart of a devil
❍ 这一回，不知怎的，～的黎元洪，脚上穿着一双厚底鞋，手上戴着一双白手套，虎起一张脸，态度很庄严。(李六如《六十年的变迁》 Ⅰ—181) …wearing a pair of thick-soled shoes and with his hands in spotless white gloves,Li Yuanhong walked in. That night his usually kindly looking face was very sullen indeed.
❍ 他这个人～，你和他打交道可要小心。This man looks kind but is cruel hearted. You must be very careful in your dealings with him.