"she's the only divine thing he's ever believed in."

And this is a concept alluded to in the original text. Euripides specifically uses/inverts Homeric imagery throughout the play, Medea’s speeches about pride and honor and how hers have been violated ring verrrry Homeric as well. Medea explicitly laments in her first appearance on stage that she was not born a man who could go through battle instead of childbirth. I think her rage is even explicitly likened to Achilles’ at one point but I don’t have my book on me to find the citation. And when she kills her kids, she doesn’t do it in the usual way she takes care of her enemies with poison (a ‘womanly’ mode of violence) she kills them w a bronze sword, AKA the very masculine weapon of heroes.

But aside from that, it’s also very important to the narrative that Medea and Jason’s kids have to die. It’s never a matter of their son’s choice bc they are both too young. Medea is forced to choose between leaving them with Jason (a man who has heartily demonstrated his selfishness and disregard for them) in Corinth where they will almost certainly be in danger from the royal family (a fear of hers that is heavily implied to be justified. Frankly all of her fears and predictions are justified), or they can come with her and live the life of fatherless and homeless exiles w barbarian ancestry in Greece, which is another horrible and painful option.

Her only escape route is that she’s suckered kindly old King Aegeus of Athens into saying he’ll provide shelter for her no questions asked. But even this is not a terribly secure option despite the oaths she made him swear. Medea (and her sons by association) is still a foreigner, still fatherless, and still not in her own homeland which is a very dangerous and insecure position for a person in Ancient Greece. And we the audience know she was right not to take her sons there with her bc she gets chased out of Athens and is homeless again when Aegeus’ son Theseus grows up.

Medea has just spent the whole play lamenting about how much pain and insecurity she is experiencing as a result of being a barbarian (aka foreigner) in Greece who does not have the luxury of family to help her or provide shelter for her. And Jason’s only family wouldn’t help them either bc they’ve burned that bridge pretty thoroughly already, that’s why they were living in Corinth in the first place. Medea explicitly says that she will not let her kids suffer, and that life would certainly be full of it.

But Medea doesn’t just make this decision for her kids wellbeing, her main reasoning is that Jason needs to be punished and that will not be complete without destroying his only children. And make no mistake, Jason needs to be punished, because his crimes aren’t just against Medea. He has broken sacred oaths and disrespected several gods.

Jason while arguing with Medea claims that he doesn’t have to be grateful to her or anyone else for helping him, except maybe Cypris (AKA Aphrodite) for making Medea fall in love with him and therefore do all the things she did to repeatedly save him. Which is… blatantly untrue. Jason is helped by several mortals and more importantly several GODS on his journeys, who are… shall we say… famously uncool about going unthanked and famously VERY cool with killing blasphemers kids as punishment.

And there is no god he has betrayed more egregiously than Hera, his own patron goddess who was with him from day one. The goddess of marriage, who presided over his own marriage, a marriage he has just betrayed. A goddess that Medea doesn’t fall out of favor with, because when she buries her children she chooses a sanctuary sacred to Hera, where she knows her children’s graves will be protected from desecration by her enemies.

When Medea kills their kids much of her reasoning is that they’ll be in danger from her own enemies and that if someone’s going to kill her kids it should be her, who loves them and gave them life. But their father has plenty of enemies too, and the gods conspicuous absence implies their work is already being done by Medea, which is confirmed when she appears on the mechane at the end of the play, a staging choice in Greek tragedy that is reserved for gods.

Jason’s kids always had to die because of what Jason did. He’s obsessed with his legacy and reputation, it’s what he betrayed Medea for in the first place because he wanted a more prestigious marriage. As long as his kids live, he still has one and therefore his punishment isn’t complete. And it is at the end. Jason is completely alone, with no wife, no children, no status, no kingdom of his own. Medea flies away in a golden chariot gifted to her by her divine grandfather while Jason cries out into the silence for the gods and gets nothing. He dies years later, alone and in anonymity, by a fragment of his old ships mast falling on his head. It’s literally the worst fate possible for a Greek hero.

The tragedy (because this is, after all, a tragedy) isn’t what happens to Jason, his is a punishment for pride, blasphemy and oathbreaking. The emotional core of the play centers around Medea’s loss, and her agony over what she has to do. If the kids don’t die then you’ve taken the tragedy out of the tragedy.