viper
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.