Sylvanas is not a Genocidal Maniac, because my own Theory is that She has no, and has never had Free Will. She has never consciously made the decisions that She makes within the game.
She was originally allowed to “slip” control by The Lich King in order to counter Arthas, who was growing too powerful, and who would have used the Scourge to actively fight the Legion. Ner’zhul did not feel, originally, that Arthas could be convinced to go to war against the true enemy of the Universe, the Void, which would require the aid of the Legion, which Arthas bore a personal grudge against. So he sought to have Arthas removed from the Scourge. But he could not do so outwardly, and still retain Kel’Thuzad as a useful member of the Scourge.
The plan changed, however, once Illidan began his assault on Icecrown. Arthas, the most powerful leader within the Scourge was recalled to Icecrown and defeated Illidan. After the battle, Ner’zhul changed his mind, and sought direct melding with Arthas to show him the Scourge’s origin from the Legion, their true enemy, and convince him to take up the battle against the Void.
Their debate on how best to fight the Void raged internally for years… until the Cult of the Damned witnessed the fall of C’thun and reported the events beneath the sands of Silithus to the Lich King.
With a renewed plan to use the Mortals to battle the Old Gods of Azeroth, everything since Wrath has been the Lich King manipulating the Living into picking off the Old Gods, one by one.
We were driven towards the Storm Peaks to discover Ulduar, the one area in Icecrown without a Scourge presence. If we triumphed against Yogg’Saron, all the better. We would prove to be the most powerful mortals for the Lich King to bind to his direct will. If we fell, then we would weaken the Old God for the Scourge to destroy.
When Bolvar took up the Helm of Domination atop Icecrown, the strategy, the enemy of all life in the Universe was revealed to him. And he offered Ner’zhul and Arthas a variation on their current strategy.
Instead of turning the Living into undead monsters, whose power would cap off… they would then manipulate the Living into War and conflict. Dragging them further and further into more desperate and dire situations that would require them to grow more and more powerful, as there is no greater motivation than the will to Live.
The Cult of the Damned steered Anduin’s ship to the Misty Isle of Pandaria, knowledge of which was given to the Lich King by risen Dragons, there to destroy the Heart of Y’sharrj, and to cripple the Mantid forces before they could unite with N’zoth’s forces beneath the seas. Just how did Garrosh uncover the Heart? Was it because of Goblin greed? Or were the Goblins tipped off to its location by Agents of the Cult of the Damned working within the various Archaeology branches of both the Horde and Alliance?
Conflict was stirred here and there, and both sides kept in check by bad intelligence here, a delayed report there that lead to a successful strike against a critical target.
As Vol’jin came to power, though, peace threatened to break out on Azeroth. So Vol’jin had to be replaced with someone who could be made to stir the conflict pot and continue to force growth through conflict among the living to combat the Void. His wounding on the Broken Shore was fortuitous, and it would have been simple for a powerful Scourge minion, like Kel’Thuzad (we never did find or destroy his Phylactery in Northrend, and he is obviously adept at hiding his presence, as we have not seen him since Naxxramas), to prevent him from regenerating and to block his access to the Loa.
Enter Sylvanas, who never had Free Will, never made an un-influenced decision since Her death. The recreation of the Blight was nothing more than the Lich King perfecting Kel’Thuzad’s original Plague formula.
Her suicide atop Icecrown was nothing more than a way to make Her believe She was bound to the Valkyr, and a way of motivating her to strengthen the Forsaken and increase their ranks. To use that perfected Plague.
After that, all the Lich King had to do was to enhance Her normal paranoia, Her normal vindictiveness, and reinforce Her Superiority complex and watch Her go.
What is more, though, is that we discover during the Death Knight campaign that the Lich King has always had the ability to influence any member of the Scourge, no matter how distant, no matter how “free”. This was commented on by several members of the Ebon Blade during the opening quests.
The Lich King even had Frostmourne Reforged, and placed into the hands of the most powerful Death Knight on Azeroth during the Legion Invasion, and his aid was only granted after the Legion’s intent to wipe out Azeroth was discovered.
So no, Sylvanas isn’t a Genocidal Maniac, because to be a Genocidal Maniac you have to make the decision to commit Genocide. Sylvanas hasn’t had free will since Her death. She is, as She always has been, nothing more than an agent of the Lich King.