Can Jason's actions in Medea be justified?

I'm not entirely sure that Jason's actions can be justified. Medea went out a limb for him and he repaid her unfailing loyalty by dumping her. Medea took enormous risks to help Jason get ahold of the Golden Fleece. Yet Jason was simply using her all along; he had no intention of settling down with her and making her his queen.

After being unceremoniously abandoned by Jason, Medea's reduced to the status of a non-person. She has no family ties in Corinth, and so has no one to turn to there. Besides, she's decidedly unwelcome in the city after letting rip with a blood-curdling scream of revenge. But nor can she return to Colchis after helping Jason to steal the Golden Fleece. Her own people will doubtless disown her as a traitor. Medea's in a real pickle, and it's all down to Jason. Thanks to him, Medea's caught between a rock and a hard place and there appears no way out of her terrible predicament.

Can Jason's actions in Medea be justified?

It is not easy to justify Jason's course of action.  Euripides metes out a great deal of responsibility for why Medea is the way she is because of Jason's actions.  There is a great deal of selfishness and a sense of self- indulgence that is within Jason's actions that make it difficult to persuade one that his actions actually can be justified.  It seems to me that the best approach to take is to argue that Medea's actions are not justified.  In this light, Jason's actions do not look as bad as Medea's.  Perhaps, it is in this condition that one could begin to search for some semblance of justification.  Medea's actions are so destructive socially and personally that she creates a threshold for unjustifiable actions that no one can approach.  While Jason's actions are selfish, they are nothing as selfish as Medea slaughtering their children.  Jason's actions cannot be justified on their own merit.  Yet, when compared to Medea's actions which embody the lack of justification, Jason's actions might be considered more tolerable.  Medea's actions are probably the embodiment of the lack of justification.  Jason's actions are wrong and bad, but are not to this level.  Absolute standards of conduct have become relative to the actions of another.  This might be one of the lasting legacies of the portraits that Euripides has offered.   I think that this becomes the standard that one has to use in order to determine whether or not Jason's actions can be justified.  

What wrongs has Jason committed in Euripides' "Medea" and what motivates him?

In Medea, Jason has done some wrong things. He dishonors the commitment he made to his wife, leaves his family, and marries someone else, even though he is already married to Medea. Even if this is acceptable in society and he has good reasons, he never gives her a choice.

Jason and Medea were married; they had sworn an oath to be together and build a life together. He ultimately decides not to honor this oath in a way that's acceptable to Medea when he decides to commit to Glauce. It's clear that, despite the things he promised her when he convinced her to marry him and have his children, Jason has no care for Medea. It's easy for him to walk away from her and commit his life to another. Leaving your wife in the way Jason did is wrong morally. He's not sorry. He believes all woman are the same and ultimately unnecessary. He wishes men could have children without women. To him, marriage is only a means to an end.

Jason also abandoned his responsibilities. He was responsible not only for being Medea's husband but for building a life with her. In their history together, she helped him achieve his goals, saved his life, and he pledged that they would be together. This isn't just marriage—it's a partnership between two people. When he leaves Medea, the goals they'd set change irreparably. Her place in society is changed. She says that "divorce is unsavoury for a woman and it is not possible to say no to one's husband."

Jason takes away Medea's choices. She's no longer his wife and will only stay on as his second woman. Medea had a good life before she left to live with Jason in Greece. She laments this choice because Jason is happy in Greece and she's far from home and left, now, with nothing. She betrayed her family for Jason. Creon, Glauce's father, banishes her for the fear that he'll hurt his daughter. Jason comes to Medea and says, "This is not the first time [...] I have seen how utterly stupid an intransigent temper can be." He points out that she could have stayed in her home by obeying him and accepting his new marriage but now has to be banished.

Jason also did a disservice to his children. He know what kind of woman Medea is and still leaves them with her when he goes to Glauce. Medea is screaming out her hatred for them and her father. She says, "You horrible children, of a mother who hates you / god damn you with your father, and the whole house go to Hell." Jason either isn't interested enough or clever enough to see that she'll use them against him, even though it's obvious in her character. He's selfish and self-centered.

Jason wants to marry Glauce for social position and money. He feels that he and Medea are too poor for him to be happy. Jason does care about his sons and wants them to be well-placed to have a good life in Greece. He says,

Now the reproaches you heap on my royal marriage,
here I will prove first that I did the smart thing
and showed good sense and finally that I am a great benefactor
to you and my children. Let me finish.
When I arrived here from the land of Iolcus
dragging with me many useless encumbrances,
what luckier opportunity could I have found
than, as a refugee, to marry the king's daughter?
It is not what is eating you, that I hated my marriage to you
and was infatuated by desire of my new bride,
and not that I had a craving for more and more children
—the ones I have are enough and I am satisfied with them—
but so that—and this is the point—we might live well
and not be in need. I am well aware
that even a friend shuns a poor man and stays out of his way,
and I wanted to bring up children worthily of my house,
and father brothers to your children
and put them on an equal footing and join the families
so that we could live well.

These motivations, of course, never come to fruition. Instead, Creon, Glauce, and Jason's children are all killed in the course of Medea's revenge.