In answering this question it's important to understand Medea's condition in its social context. Ancient Greek society was deeply patriarchal; women were without economic or political power and expected to serve the needs of their menfolk without complaint.
Medea, however, is a transgressive figure in that she defies her father by helping Jason steal the Golden Fleece. In doing so, as she frankly acknowledges, she is betraying her father and her family. Not only that, though: she's attacking the very foundations of patriarchal society.
Having stuck her neck out for Jason, one can only imagine Medea's horror when the man she thought truly loved her dumps her for Glauce, the daughter of Creon, King of Corinth. Now that she's been abandoned, Medea has nowhere to turn and no one to turn to.
After betraying her family by helping Jason to steal the Golden Fleece, she can't very well return home. And as a "barbarian" woman in Corinth without any kind of social support network, staying put is not really an option either. In any case, Creon has decided to banish Medea, so she wouldn't have been able to stay even if she'd wanted to.
On a feminist reading of the play, then, Medea is just one of many female victims of the patriarchy and its warped moral values. Yet unlike most other women in ancient society, Medea hits back at the patriarchy, albeit through morally reprehensible means.
Feminist critics don't attempt to justify any of Medea's repulsive actions, but they do tend to claim that such actions represent an attempt to challenge the values of the patriarchy, those very same values that have turned Medea's life upside-down, forcing her into taking such drastic measures to regain control of her life.