I don't know, keeping a pet dreadlord around it felt like just a matter of time IMO until Varmatharas pulled something like that. They didn't need to involve the Legion to make it dual faction, having Putress attacking both factions at the Wrathgate could have done that on its own. The legion invasion was a consequence of Sylvanas falsely believing she'd cowed Varmatharas and not keeping a close enough eye on him.
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I wouldn't say they were more involved than the forsaken who ordered the deed done. Though I suppose she was acting as warchief of the entire horde at that point not just as a forsaken.