a viper in (one's) bosom

A friend, lover, or relation who proves to be traitorous, untrustworthy, deceitful, or ungrateful. (Used especially in the phrase "nourish/nurse/nurture a viper in one's bosom.") Well, it turns out that Margaret was quite a viper in my bosom. I put my neck on the line to get her a job in our company, and then she turns around and tries to steal my position! I thought our love was not only mutual but indestructible; and yet, I have nursed a viper in my bosom all these years: my darling husband has cast me out and run off with a younger woman. I thought the profligate had seen the light and was seeking redemption, and so I took him into my care. But before long, I knew I had nurtured a viper in my bosom, as I awoke one morning to find myself robbed blind!

a viper in your bosom

a person you have helped but who behaves treacherously towards you.

The phrase comes from one of Aesop's fables, in which a viper reared in a person's bosom eventually bites its nurturer. The idea is also found in Latin (in sinu viperam habere ) and the expression appears in various forms in English from the late 16th century.