@Doomer
Because he's a decent person, and isn't used to being what amounts to a god.
Think what happens if he shows up on his own without a powerful faction behind him? His own faction would be nothing compared to him, but the important bit is that it's a power the rest of the world can understand, like their own. If he shows up on his own, he's 'free power'. "Maybe if we take enough dudes, we can make him ours." And then what happens? He's forced to kill armies? Thousands and thousands of people with families, forced to do the bidding of their owners and being told it's for the sake and safety of their people?
Then after he gets his first victories in, murdering thousands of innocent people to save his own single life (not a fair trade), everyone bands together because 'this is a threat to all of our kingdoms/countries/all life on the planet'. And it becomes him vs. the world. During which they're still doing their best to become the last man standing with all of his power.