See, now, to me, a 'sin' in the context of Warcraft, and particularly the Horde and their faction doctrine, is a betrayal of the absolute leader, the Warchief. And, as far as I can recall, Sylvanas has made no betrayal of the Horde which has actually come to light, at least, not beyond High Warlord Cromush, after his stationing in the Eastern Kingdoms.
Her one potential sin, committed against the current Warchief, would have been the use of the plague and necromancy after Garrosh explicitly ordered her not to. This was, I believe, never made known to Garrosh and I think it is because after being stationed with the Forsaken, Cromush began to understand how fundamental Sylvanas was to their political/military structure, their culture
and, most importantly, keeping them somewhat in check. I believe this revelation is articulated in his frantic, barked order at the Valk'yr, demanding they, "Fix her...FIX HER!", when she is shot by Crowley.
I think, after he saw how she operated and undeniably got results (despite being temporarily set back by a bullet in the brain!), he decided to keep schtum on any tactics she might have utilised which the Warchief might have disagreed with. (Whether he did actually rat her out is not known yet, could change, but I work with what I've got.)
Of course, she has done many cruel, cold, malicious and downright wicked things recently. But, we have to remember who to, it was the humans. The enemies of the Horde. The
enemies in an
open war. These are not despicable tactics used against unsuspecting targets in covert operations, something which both factions were guilty of during the unspoken cold war between the Alliance and Horde before the Wrathgate. This is open war, one which Varian, a leader on the other side declared. Some of the things she has done have been terrible. But, it wasn't done to the people on her side (not since WC3-ish time). She's done well by the Horde. Got them a port, made their lands in the Eastern Kingdoms more secure and kept the Alliance forces in Eastern Kingdoms too busy to attempt to take much more of Kalimdor (imagine the Barrens fight if Sylvanas wasn't causing trouble on the other side of the pond.).
Her moral 'sins' are many. But, for the Horde, it shouldn't matter (also worth noting, many of the terrible things she has done have only been seen by we, the omniscient players, and a few characters in game. The majority of people, both sides, are not privy to the insight we have).
But, if it is sins committed against the Horde which we are talking about being forgiven, with the reinstatement of Thrall as top-dog, Sylvanas has none to redeem for (which most people know about
).
Of course, Blizzard might feel the same way many people seem to do. That she must repent for making an enemy suffer. Something which seems ludicrous to me, but they could very well choose to think that way, especially with making Thrall Warchief again and bringing back Horde Light. >_<'
Hope that made sense. I waffled. (Oh Lawdy! Wall of text. I'm sorry but it's a discussion I enjoy contributing to!
)