During the Tang Dynasty (618-907), there was a minister named Li Yifu. He was skilled at flattery, which helped him rise to the position of Prime Minister. Despite his outwardly sweet smile in conversation, Li was a vicious man who often conjured up evil ideas in his heart.
Once, upon hearing that a beautiful woman had been imprisoned, Li summoned the prison officer and ordered her release. The officer complied, and Li took the woman home.
Later, when the case was reported to the emperor, the prison officer grew fearful, knowing he had committed a crime by freeing a prisoner. He sought Li Yifu's help, but Li ignored him and told him not to cause trouble. In despair, the officer hanged himself.
Another official, learning of this tragic suicide, intended to reveal the truth to the emperor. However, Li discovered the plan and made false accusations against him. The gullible emperor believed Li and exiled the official to a remote region.
This story is the origin of the idiom used to describe someone with murderous intent behind a smile.