This is one of those times when we're talking about the Warlords version of a character, and not the original. This is not the Ner'zhul we know - this is not the elder shaman who united the orc clans while being manipulated by Kil'jaeden. Although this Ner'zhul is certainly a respected shaman and the leader of the Shadowmoon Clan, he's not the same as the orc we know - he hasn't made the same choices. And yet, he is Nerzhul. The deathspeaker, the shaman who watched the stars and listened to the stories they told him.
On the Draenor we now explore, history has unfolded differently. Some things are the same - Ner'zhul's student turned on his master and sold his soul for demonic power, and attempted to lead the orc clans as a whole into slavery and a genocidal war against the Draenei. But on this world, Ner'zhul wasn't so easily deceived. There are differences between the worlds even before Garrosh Hellscream's arrival - and those differences, great and small, have led to a different Ner'zhul, who made different decisions, and brought about a dark destiny for the Shadowmoon Clan. Was it worse than the rise of the Old Horde? In some ways, of course it couldn't be - at least the orcish clans of Draenor still own their own souls.
All, perhaps, except Ner'zhul and the Shadowmoon.