The Enuma Elish, the Babylonian text that tells of Tiamat's defeat and dismemberment at the hands of Marduk to form heaven and earth, is a considerably later composition than the older Sumerian religious traditions that preceded it, and Assyriologists generally read it as much as a work of political theology as a cosmogony. Its central purpose appears to be establishing Marduk, the patron god of Babylon specifically, as the supreme deity of the entire pantheon, a claim that mattered because it justified Babylon's own political primacy among Mesopotamian cities at the time the text was composed and promoted.
Earlier Sumerian creation traditions do not feature this particular combat myth or this particular hierarchy, which is part of why scholars treat the Tiamat and Marduk story as a Babylonian theological program layered onto older material rather than as the original or most ancient Mesopotamian account of creation. Tiamat herself, described as the primordial saltwater ocean and mother of the first generation of gods, predates Marduk's story considerably as a cosmological concept, and her transformation into a monstrous enemy to be violently defeated specifically serves the text's larger argument for Marduk's supremacy, making her villainy in the received version a function of Babylonian political needs rather than a fixed trait carried unchanged from the earliest Mesopotamian religious thought.