Mind you, this is my two coppers on the matter, but I think a big issue is how dismissive of Horde Culture we can be when we discuss moral responsibility and other similar topics, particularly in relationship to the actions of the Horde and the PC.
The Horde isn’t the Alliance.
The Horde isn’t Modern-Day Earth.
It’s been discussed in the past, but morality is more within the realm of the Alliance, whereas the Horde is more concerned with the realm of Pragmatism.
In the Alliance, if you steal, you’re wrong. Doesn’t matter that you’re feeding your family, or giving to the poor. Stealing is wrong and you are a criminal for doing it.
In the Horde, if you steal, you might not be wrong. The greater crime would be letting your family starve, or letting other parts of the Horde suffer. So long as the theft serves a pragmatic purpose, it can even be considered, ‘honorable.’ The Cooking Dailies in Orgrimmar come to mind where the Orc Chef has you steal Darkspear Rice to feed the soldiers.
Looking at things through the lens of the Horde’s culture of Pragmatism, the destruction of Teldrassil is not an inherently bad thing worth sparking a rebellion over. Same way the Destruction of Theramore was not worth sparking a rebellion. Horde interests were being pursued. What happened was objectively terrible, but because it served the Horde, no one within the Horde can really speak out against it. That’s just not how the Horde works.
When Derek Proudmoore was raised into Undeath, he became one of the Forsaken. The Forsaken are part of the Horde. Therefor, by actively violating his Free Will in order to create a weapon, Sylvanas was acting against the Horde. This is not unlike Garrosh’s assassination attempt on Vol’jin.
In terms of morality, the Horde is a selfish, self-serving, hypocritical force of pure destruction.
In terms of pragmatism, the Horde is a tight-knit group united by their need to survive.