Wow. To me, that's so far off the beaten path that who knows where to begin with it-- you've got religion in there somewhere, as well as the old question of our existence itself. But supposing that were true (and it's almost too big a jump for even me to make, except for the fact that even if it is true, we'll likely never know it), what would you do if you were living in a virtual world? Would you act differently? Would you be a griefer? Or would you play the game, play by the rules, and help yourself and others not only "win," but have fun too? It seems a little loony (because who wants to admit that their world is someone else's toy?), but it's a fascinating thought experiment that should help you examine both how you're living your life and how you want to live it-- what would you do if you were an NPC in a virtual world?
Of course, things get even stranger, because we don't just create virtual worlds-- we play in them. Azeroth isn't just full of NPCs-- it's full of us, walking around, killing things, and generally taking (and taking over) whatever we want. If our world really is someone else's, does that mean they're here too?