Hide a dagger in a smile（笑里藏刀）
During the Tang Dynasty (618-907), there was a minister named Li Yifu. He was very good at flattering people, so his official position was raised higher and higher and he eventually became the prime minister. However, Li was a vicious man. When he talked with people he always wore a sweet smile, but he was conjuring up evil ideas in his mind.
One day, Li learned that a beautiful woman was put in prison. He sent for the prison officer and ordered him to set the woman free. The man followed the order and Li took the woman home.
Later, someone reported the case to the emperor. The prison officer was very afraid because he knew he had committed a crime by setting the criminal free. He went to Li for help, but Li ignored him and asked the man not to disturb him anymore. The man was so disappointed that he hanged himself.
Hearing the tragic suicide of the prison officer, another officer wanted to reveal the truth to the emperor. Li, however, knew of this plan and began to make false accusations against the officer. The foolish emperor believed Li's words and exiled the officer to a distant land.
The idiom "hide a dagger in a smile" has come to describe a person with a murderous intent behind his/her smile.
xiào lĭ cáng dāo