And maybe there could be some romance between heaven and hell (not likely, but possible), and then they betray cause of love or something. Or people could just betray if they feel like it. Plus, think of it like this. The longer someone waits for the RPG to start, the more likely they won't participate. Since most of heaven's side signed up toward the beginning of the RPG, they are more likely to not participate than those in hell, who signed up later.