|Yes, I jumbled, middle-aged human error.
You didn't say Greek tragedies by name, but when talking about Greek plays, we generally are either talking about the tragedies or the satires, so I took it as implied. Of those, one carries moral ambiguity and that ambiguity generally leads to tragic ends, and the other is never meant to be taken seriously (or perhaps literally?), and thus it can say things about moral ambiguity that a serious play can't.
Since the Greeks are the template of Western theater, Shakespeare's tragedies are derived from Greek tragedies and follow many of the same "rules". And those rules then carry through to WSS, where whoever Tony kills, what matters is that Tony himself is punished, because we want to restore the moral order (and musical theater is tightly wrapped up with American ideas of morality). In DEH we get the opposite: someone who does something morally wrong and gets rewarded for it.
And those so-called rules aren't hard and fast. Even some of the Greek tragedies don't have a "tragic flaw", and I think you astutely point out other ways that we can understand tragic momentum on stage.