a serpent in (one's) bosom
Someone whom one has befriended, taken care of, or treated well but proves to be traitorous, untrustworthy, deceitful, or ungrateful. (A less common variant of "a viper in one's bosom." Used especially in the phrase "nourish/nurse/nurture a serpent in one's bosom.") Well, it turns out that Margaret was quite a serpent 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 get me fired! I thought our love was not only mutual but indestructible; and yet, I have nursed a serpent in my bosom all these years: my darling husband has cast me out and run off with a younger woman.