Those are the two choices though - follow Sylvanas and use the Blight or don't follow Sylvanas and focus on saving people like Saurfang tells you. You have an actual choice on who's example you want to act on. Also, that's a load of bullshit. Saurfang chooses to leave and she tells him that if he really does care more about his honor than the long-term survival of the Horde he should leave and if he does care for the Horde he should stay. She says it in a bitchy way, but that's the gist of it. So he leaves. She didn't abandon him, he chose to go. She tells Baine to fuck off because Baine was there and knew this, but is still whining.
Baine is indeed staying for the good of what he wants the Horde to be, and also because he has no spine. Nathanos I think was just affected by beign given the weapon because he was afraid for Sylvanas' safety.Baine says no because he knows he can't leave the Horde in her hands, and to go off and die would mean one less person to oppose her. What Nathanos is up to, I hope we see soon because that was some shady shit there at the end.
This is seriously one of the cases where the actions require very little justification. The soldiers are dying anyway and not using the Blight in that context would risk the lives of everyone inside and kill these people. More than that her action works, Anduin is pushed back and he and Genn are desperate before Jaina shows up to save their asses. This is actually morally grey, unlike genocide. It's far more justifiable than Illidan slaughtering his compatriots because he forgot to pack a mana potion and we spent a whole expansion hearing about what a great guy he was.