Symbolism and Mythology
According to Dorothy Dinnerstein’s book, The Mermaid and the Minotaur, human-animal hybrids such as the minotaur and the mermaid convey the emergent understanding of the ancients that human beings were both one with and different from animals and that, as such, humans' "nature is internally inconsistent, that our continuities with, and our differences from, the earth's other animals are mysterious and profound; and in these continuities, and these differences, lie both a sense of strangeness on earth and the possible key to a way of feeling at home here".
The oldest form of the mermaid in mermaid mythology is the goddess Atargatis from Syria. A famous statue of Atargatis shows her as a woman from the waist up and a fish from there down. All sea goddesses inherit the sea's qualities. Just as the sea could be gentle and nurturing or violent and deadly, so could they. These are the same contradictory qualities we see in mermaids to this day: beautiful, cruel, tender, loving, destroying, etc. In a larger sense this is man's view of nature. The mermaid, a fantastic creature, is nature herself.
Source: Wikipedia