So you want them to cease to exist after they got their vengeance? While LK was their primary goal, the Alliance threated them like shit since the moment their race appeared on Azeroth, even though they knew that they arent bound to the LKs will anymore. And honestly, even if they hated what they become it doesnt mean that they wont be satisfied with their transformation, the Forsaken who get ressed and dont go back into their graves are very well okay with it. Also, DKs dont seem to hate what LK made out of them, he wanted to use them as gun powder against Tirion, which they got their revenge for, but over all they're happy being DKs.
From any point of view, the Forsaken are the weakest because they cant reproduce. Compare them with:
- Night Elves: used to be immortal, most of them are over 10,000 years old if not older. They got Priests and Druids (along with their demigods) sects on their side, which power they feed off. They call uppon tree soldiers during war - doubt those give a damn about some gas thrown at them.
- Dreanei: I dont know if they're immortal, but if you travel 25,000 years through the galaxy you have kind of be so. With the Naaru and Velen on their side they arent exactly weak.
You simply cant compare any other WoW race to this two. Blizzard is holding their true powers back in favour of not causing more lore fuck ups.
Could we stop accusing someone of war crimes? Its a purely objective thing in WoW - dont use RL logic on WoW. Also, Varimathras actually proved his loyalty by killing one of his brothers (thats a terrible no-go for Dreadlords).
Using that logic, you could blame Thrall for leaving the Warchief position to Garrosh or any other similiar situation ever happened in WoW.
What consequences other than death for the opposite faction are we actually talking about?
She has a fanbase because of her wastly development background and story. Actually, one of the few faction leaders who're doing something. Also, she fought against the LK 1v1 in WotLK and her Forsaken wiped out the Scarlette Crusade in Dragonblight. She is cunning and clever and using the Plague as a weapon is the most efficient way to not waste more of her already low population.
Doubt is more honorable to get killed by a mace breaking your head or an arrow penetrating your body over being turned into slime by the Plague. Forsaken are different than your usual living race and she wouldnt have to bolster her defenses if humans accepted them as what they are.