I wonder the real reason Tyrande spared Varok Saurfang was to honor her past friendship with Broxigar, Saurfang's older brother. 
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Broxigar's real brother wouldn't throw an axe at Malfurion's back in the middle of a 1v1, when it's against someone notorious among the Horde even before MoP for being borderline genocidal and prone to mass slaughtering innocents.
He did instantly regret it. The heat of the moment adds a little more to the mindset of our boy Varok. Although in the game he regrets it and in the books, he fails to kill Malfurion because of Elune. Not sure what they'll make canon since in the past they ignore book lore.
