Not trying to get in the middle of this, but I think that’s new(er) lore?
I’m playing through Warcraft 3, and it sure as hell seemed like purging the city was the only option, and no one in Arthas’s crew could even suggest a better idea. Jaina isn’t totally against it (she later says Arthas is just doing what he thinks is right), she just couldn’t watch him do it.
In the game, If you fail to purge the city in time, you end up overrun and get a game over. It really gave me the impression that, if Arthas didn’t purge the city, things would have ended up even worse.
It really ties into the experience of playing the game, and I think it was intentional. Playing Warcraft 3 feels like going from an innocent fairytale world to a gritty cosmic horror story. I thought the whole point was that Arthas was being forced to make decisions that the people of Azeroth are wholly unprepared to make, but he’s convinced are fully necessary. At first, he’s correct, but he slowly keeps pushing the envelope further and further until he can’t tell right from wrong anymore.
Azeroth is a planet where right and wrong are normally simple. The whole point of Warcraft 3 was to upend that, and make the simple people of this high fantasy world deal with serious and complicated issues. Arthas is what happens when the King Arthur archetype is forced to deal with things like biological terrorism and alien invaders.
He has main character syndrome. Everything he’s ever done has been the right decision up until the third war. He starts adapting to this changing world, and has no way of realizing his perspective has gotten out of whack. After all, he’s the hero, right?
Until the moment he betrays his mercenaries, lies to his men, and chooses to watch Muradin die while he picks up the sword that steals his soul.
In your version of the story though, I totally agree. Screw Arthas. 