viper in one's bosom
Also, snake in one's bosom. An ungrateful or treacherous friend, as in I got him dozens of freelance jobs, and then he told everyone I was a lousy musician—nothing like nourishing a viper in one's bosom. This metaphoric expression, often put as nourish a viper (or snake) in one's bosom, comes from Aesop's fable about a farmer who shelters a snake dying from the cold, which then fatally bites him after it recovers. It was referred to by Chaucer and Shakespeare, and appeared in numerous proverb collections.