The Hen and the Swallow.
Reading edition
A Hen finding the eggs of a viper, and carefully keeping them warm, nourished them into life. A Swallow observing what she had done, said: "You silly creature! Why have you hatched these vipers, which, when they shall have grown, will surely inflict injury on all of us, beginning with yourself?"
If we nourish evil, it will sooner or later turn upon us.