I think it's not so much the action itself as the attitude presented. Arthas makes the call to murder countless numbers of his own citizens frighteningly quickly. He turned out to be right that the people cannot be saved, but what if he wasn't? And then he calls Uther a traitor for not immediately and unquestioningly obeying his command to massacre the city. He might have been too Lawful Good to do what's necessary, but I don't think he's wrong for second-guessing Arthas's gung-ho decision to damn so many of his own people.

And turns out, Uther and Jaina were right: Arthas had gone off the deep end. And that was the plan all along. His pride and his pragmatic willingness to abandon morality made him the Scourge's greatest pawn. Absolute power corrupts absolutely and all.

I know this is meant to be Arthas's most heinous moment but what alternatives could he have done exactly?

Neither Uther nor Jaina made any suggestions.