That was the original horde who were recruited by the burning legion to get revenge on the draenei who followed Velen instead of Kil’Jaeden and Archimonde.
They were then brought to Azeroth by the avatar of Sargeras where they were defeated, imprisoned and enslaved by the alliance. Till thrall escaped enslavement and found that shamanism could cure the lethargy caused by the lack of demon blood and started a new horde by freeing the orc internment camps. He then saved the trolls and tauren while trying to find there own land where they could be away from the alliance. That is the horde of vanilla.
Actually, the purge was an example of Alliance being good. By killing these people who were going to turn into the undead NO MATTER WHAT Arthas saved many of them from just being a wandering corpse and how painful it is to turn into one. The purge was a good thing as basically everyone was already infected and going to turn into the undead no matter what. Of course what Arthas then went to do was none of the Alliances fault as Ner Zhul and the legion are quite manipulative. (and they used his will to save his people against him)
It was morally better and strategically better. Would you rather a clean death or be turned into a corpse in an extremely painful way to then be forced to kill your loved ones.
Um no… he quarantined and then purged a city full of infectious people who were about to become mindless enemy soldiers.
Arthas did nothing wrong at Stratholm, that was actually the best option available to him.
Oh and the forsaken were an ally of convenience, they needed allies to help kill Athas so Sylvanas could get her revenge and the Alliance hates the undead so thats a no go, then you have the horde who could use a foothold on the Eastern Kingdoms so allies of convenience.
Indiscriminately murdering people because there is bad grain in the city is nothing but mass murder.
It was the quick and easy path, because Arthas was a hothead who didn’t think things through. There was a reason Jaina and Uther the Lightbringer walked away, and it wasn’t because Arthas was doing a “good” thing.
It wasn’t strategic and it wasn’t “morally better.” It was impetuous and evil. Innocents died because Arthas couldn’t wait for a better plan.
Regardless of the wrong horde or not, at the time, the true nature of the Orc was shown, and they clearly knew what they were doing wasn’t the true intent of their ancestors.
Yet, despite all the signs of their wrongdoings and the dark magic around them, they still succumbed to their lust for power, even if it meant the slaughter of innocents.