Her change from protective, nurturing mother to vengeful warrior-queen has been claimed to be due to a shift in the theological paradigm in Mesopotamia during the reign of Hammurabi during which female deities (and women in general) lost their status. This interpretation was first suggested by the British scholar Robert Graves and later popularized by historians such as Merlin Stone in the bestselling book When God Was a Woman.
The argument claims that Tiamat combines the earlier qualities of Nammu as a Mother Goddess with the later association of female deities as troublesome and vindictive, as in the case of Inanna who becomes the erratic and violent Ishtar by the time of The Epic of Gilgamesh, flies into a fury when Gilgameshrejects her advances and causes the death of the Bull of Heaven and Gilgamesh’s friend Enkidu. Graves, and others, claim that the shift from a matriarchal to a patriarchal theological vision is poetically addressed in the figure of Tiamat in Enuma Elish because it could not be overtly stated in any other form.
There is no doubt that the worship of female deities declined during Hammurabi’s reign and formerly powerful goddesses were replaced by gods. The problem with Graves’ interpretation is that there is no evidence of there ever having been a “matriarchal theological paradigm” in Mesopotamia. The earliest Sumerian inscriptions concerning the gods, while they do mention many feminine deities, always have a dominant male presiding over the Mesopotamian pantheon.
Excerpt from : https://www.ancient.eu/Tiamat/