One of Prophecy's central themes was the idea of coming to terms with survivor's guilt: Casey blamed himself for Blair's abduction, Hawk needed to revenge Iceman, Maniac couldn't deal with sending men under him out to die...
And then Blair -- the Nephilim captured him and, as he put it, forced him to remember things: Tolwyn, Kilrah, Angel and so forth. Blair was directly responsible for billions of deaths, and had watched almost everyone he felt close to die when he lived. I submit that he had no intention of coming back -- he went out to end his suffering (he had been trying to get assigned to fly ever since he'd returned from the centrax), and he happened to get a chance to save the universe (and in so doing he achieved the ultimate, more obvious purpose of the game -- passing the torch on to Casey, a clean slate).