A contentious topic for the Horde in particular, it’s often used as justification for the genocidal war perpetuated by Sylvanas in BfA and seeming violation of the truce, but it might just have saved Azeroth in the long term although how might not be readily apparent.
So why did Genn go to Stormheim to begin with? Because according to SI:7, Sylvanas herself had taken a significant portion of the Forsaken military there in search of something and was acting particularly secretive about it. And regardless of who gave him that information or the intent behind it, it was actually correct: she WAS up to no good.
Let’s be clear. Sylvanas did not go to Stormheim to enslave Eyir in order to benefit the Horde, she did so in order to gain functional immortality and nigh unlimited power for herself. And while such power might have benefited the Horde in the short term and sped up the Legion’s defeat, it would have caused even more catastrophic consequences had Genn not stopped it from happening.
With unlimited Val’kyr and the ability to mass resurrect potent undead, Sylvanas would no longer require the Horde whatsoever as she would now possess a power even greater than the Scourge at its height. Not even the Lich King himself had access to such power, being forced to individually create Val’kyr on a case by case basis and never in such numbers. Let me repeat that: not even the Lich King had this power at his command.
Did the Horde playerbase not find it odd that her first act as Warchief was to find a means to make them completely unnecessary to her continued existence and that of her people? And if she actually gained that power, do you truly believe she would have been content to remain answerable to the Horde leadership?
And worse, even the Lich King largely restrained the Scourge from wreaking havoc across Azeroth because a sliver of goodness remained inside him. But Sylvanas? She has no such goodness or restraint. Had she succeeded, I have no doubt whatsoever that she would have immediately turned such power onto the Alliance to ensure no threats remained to her existence as soon as the Legion was eliminated.
Some of the Horde might be convinced to go along with it at least initially. But after seeing Sylvanas go through with her threats like raising all of SW as her slaves, even self absorbed Horde leaders like Gallywix would quickly realize the danger to their own existence she represents were they to step out of line and would be forced to band together with the remainder of the Alliance to combat her, and even then the chances of victory would be slim.
With uncountable undead at her command and unlimited Val’kyr to raise every fallen corpse as well as create ever more potent forms of undeath, there might have been no stopping her and Azeroth itself may very well have been turned into a barren wasteland populated only by shambling corpses bound to her will.
Now Horde players might just call this propaganda and paranoia, but really ask yourselves: does Sylvanas truly consider the Horde her family, or a necessary evil to keep her people and herself safe from the myriad of foes that she knows would destroy her without their protection?
You may hate Genn, but he may have very well saved you as well as everyone else.