Medea is a very complex character, which is what makes her such an endless source of fascination. Neither completely heroic nor totally villainous, she has both good and bad sides to her character.
First, let's talk about the good sides. One can only admire the way that Medea shows strength and fortitude while making her way in the world. Medea lives at a time when women are expected to do what their menfolk tell them to do. In ancient Greece, women were confined to the home, where they spent all day doing household chores, raising children, and spinning away on their distaffs. For a woman in ancient Greece, there was no world outside the home to speak of.
Yet Medea didn't get the memo. Not only does she venture outside the house and act heroically; she also defies the men in her life, most notably her father and her husband Jason. It is such behavior that has earned Medea the status of a feminist icon. Feminists have also warmed to Medea for overcoming seemingly insurmountable odds and making her own way in the world despite being both a woman in ancient Greece—with all the associated social handicaps—as well as a stateless refugee without any kind of family support network.
Now let's look at Medea's villainous side. Medea notoriously kills her children as a way of gaining revenge on Jason for dumping her. Although we might sympathize with Medea for wanting to get some payback on the man who treated her so abominably, this is most definitely not the right way to do it.
Much the same could be said of Medea's brutal murder of Jason's new wife, Glauce. Again, it's perfectly reasonable for Medea to feel some resentment towards the woman who effectively stole her husband away from her. But once more, the method of revenge is morally repugnant. Glauce dies a pretty horrible death after putting on a poisoned robe given to her by Medea. Whichever way you look at it, this is villainous behavior by anyone's standards.
On the whole, you'd have to say that there's much more of the villain in Medea than the hero. But there's just about enough of the latter to make her a morally ambiguous figure overall.