How can I describe Medea as a mother, a child killer, a sorcerer, and as a woman, and how can I describe her aims?

What a great question. Medea is a terrifying figure and the very fact that the ancient Greeks portrayed her as a witch tells us something. In short, the Greeks slant her in a negative way right from the beginning. A more interesting perspective is the view of Ovid, the Roman poet. He knows the ancient tradition and stories of Medea, but he portrays her in a much more sympathetic light. I highly suggest you read the section on Medea in Ovid's Metamorphoses - book 7. These lines show the agony of Medea. She actually has a voice and we can see that she is gripped with love, longing guilt, loyalties, etc. Also when we realizes that Jason later scorns and betrays her love, she acts in a certain way. Is this right or wrong? Whatever the answer is, there is always a context. And this context will give you enough information to form a sophisticated argument.