Campione!

From MangaHelpers: In the world, especially during ancient times, gods existed and, even now, find their place in the world. But at times, there are mortals who, whether intentionally or not, kill one of these gods and receive the "curse" of the name "Campione" or God Slayer. Those under this curse gain the power of the god they slayed and receive the mission to obliterate any false god who attempt to take control...