Medea is a play totally lacking in heroes. Jason is an unfaithful husband, willing to throw Medea aside for another woman, despite Medea's severing all ties with her former life in order to be with him. However, Medea herself is far from a noble tragic heroine: she kills many people without remorse, including her own sons, and then ends the play in miserable exile, never gaining the self-awareness most classical tragic heroes do.
Medea is also uninterested in virtuous conduct once she has been wronged. She mentions as much when plotting her vengeance:
Up, then, Medea, spare not the secrets of thy art in plotting and devising; on to the danger. Now comes a struggle needing courage. Dost see what thou art suffering? 'Tis not for thee to be a laughing-stock to the race of Sisyphus by reason of this wedding of Jason, sprung, as thou art, from noble sire, and of the Sun-god's race. Thou hast cunning; and, more than this, we women, though by nature little apt for virtuous deeds, are most expert to fashion any mischief.
The most shocking part of Medea's revenge plot is that she gets away with it. She murders without a second thought and escapes legal justice at the end of the play. In most tragedies, it is the will of the gods that justice be done on earth, but here, with Medea escaping punishment, the implication is either that the gods are absent or they are on Medea's side. And if they are on Medea's side, then they are uninterested in providing justice for her sons or the other innocent people whose lives she destroyed. The leader of the chorus claims that the gods are heaping vengeance on Jason through Medea's actions ("This day the deity, it seems, will mass on Jason, as he well deserves, heavy load of evils."), but this is still questionable justice.
Ultimately, Euripides is presenting a morally gray world in which justice seems an elusive thing, neither provided by humanity or the gods. While the guilty Jason suffers, the guilty Medea goes unpunished for her actions and the many innocent victims of her vengeance are themselves unavenged by the play's finale.