Any citations for this the explanation of Mitsuhide's betrayal? As far as I knew, no one knows the reason and whether it was the treaty, Oda simply calling him names, or fear he was being supplanted as an important general (or other factors).    The massacre at Mt. Hiei was brutal, but Nobunaga was trying to break the militancy of the Buddhist monks  (Sansom has a good thorough line about this in his three volume history). Buddhist monks through much of Japanese history were a large, well-armed faction that caused problems for both the court in Kyoto and for daimyo. Without crushing them to the roots, its likely they would have continued causing problems for those groups.