The Scourge were led by an orc, so pretty much any claim against a corrupt Arthas falls back to the Horde up through the end of TFT. WotLK is allegedly entirely Arthas in there, no longer Ner'zhul, but before that, Ner'zhul is the one who was influencing Kel'thuzad and controlling the undead, including Arthas, if Sylvanas' claims of mind-control are to be believed. Given how much human Arthas was a contrarian to his superiors and how much a death knight Arthas was submissive to the Lich King, I see no reason to suspect Sylvanas was lying about that.
But as you point out, blaming an entire faction is pointless. Both Alliance and Horde have done a lot of damage. Does it really matter the degrees? I think I've very rarely seen an atrocity list comparison result in members of either side professing a change of opinion about the topic.