It seems like Greeks had a thing for bringing their heroes to a bad end. Heracles, Theseus, and many more all achieve some pretty awesome things, but in the end die kind of a pathetic death. Jason joins the ranks of his fellow fallen heroes when his head is randomly crushed in by a rotting chunk of the Argo after many uneventful years of unhappiness and depression. Wow, what kind of reward is that for the hero who brought back the Golden Fleece?
Like many of his hero buddies, Jason's downfall comes up because of his treatment of a woman. When Jason betrays Medea by marrying Glauce, the Princess of Corinth, Medea takes a horrible revenge on him by killing their two sons, Glauce, and Glauce's father Creon. This reminds us a lot of Heracles, whose wife, Deinara, accidentally poisons him in an attempt to keep him from leaving her for another woman. Unlike Heracles, Jason doesn't die quite yet, but instead lives on, feeling lonely and miserable. He never gets to be the king of anywhere, and everything else in his life is awful because the gods are mad at him for betraying Medea.
So, what's the moral here? Hmm, how about "don't cheat on your wife or she might kill your children"? That's a possibility. Or maybe... "it's awesome to be a hero, but anybody who rises to great heights is bound to fall"? You see this theme all through Greek mythology, when heroes become guilty of hubris, or overweening pride, and are destroyed because of it. Could it be that it was hubris that allowed him to think he could betray Medea and get away with it? Like in many of the great tragedies, Jason is inevitably brought down despite his greatness... or maybe because of it.
For much more on Jason, check out "Jason: Birth and Early Adventures" and "Jason: The Quest for the Golden Fleece."