The Lore surrounding Lamashtu
There are old stories explaining Lamashtu’s evil nature. She was the daughter of the skygod Anum and the sister of Ishtar, the important goddess of love, fertility and battle. Lamashtu was thus also a goddess, but one with unusual preferences. She wanted to eat human baby flesh for dinner. Because of this, her father exiled her from the heavens. Down on earth she maintained her bloodthirst by preying on infants and other vulnerable people, like women giving birth and the sick.
Another explanation for Lamashtu’s existence has to do with the Mesopotamian Flood Story. After the Flood has swept across the earth and the Floodhero has survived with his family, the gods decide that they want to keep the growth of the human population in check. One of the ways to do this was through crib death caused by creatures like Lamashtu.