I doubt that Willow will be set up as Buffy's future enemy, but lying, cheating? Morally bad yes, but as bad as considering killing maybe lots of people not just Warren (who, until Joss was called up on it, we were supposed to believe that Amy had saved before he died, therefore, Willow never killed anyone, no matter what the intention), who killed her lover, which could cause a case of temporary insanity? That Willow?