a snake in (one's) bosom
Someone whom one has befriended, taken care of, or treated well but proves to be traitorous, untrustworthy, deceitful, or ungrateful. (Used especially in the phrase "nourish/nurse/nurture a snake in one's bosom.") Well, it turns out that Margaret was quite a snake 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 snake in my bosom all these years: my darling husband has cast me out and run off with a younger woman.