nurture a serpent in (one's) bosom
To befriend, look after, or take care of someone who proves to be traitorous, untrustworthy, deceitful, or ungrateful. (A less common variant of "nurture a viper in one's bosom.") 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 serpent in my bosom, as I awoke one morning to find myself robbed blind! I thought our love was not only mutual but indestructible; and yet, I have nurtured a serpent in my bosom all these years: my darling husband has cast me out and run off with a younger woman.