Not necessarily. Attila was already in no good state when he encountered the pope (Italy isn't good country for cavalry armies at the best of times and he had caught a bad time). The stories about persuasive powers or heavenly apparitions are nice, but hardly convincing. That would mean he'd only do that if the pope manages to insult him badly enough. from what I 've read, Attila took a lot of provocation. That means he's irrational and will attrite his suffering army in Italy (attacking Rome, a purely symbolic target untenable if taken). A decisive Roman victory becomes likely at this point. The City will elect another bishop and Leo likely be canonised as a martyr, and all is well with the world.