Bloodstone

Three centuries after a huge flood caused by human technology reduces civilization to a replica of the 19th century American West, a wounded, amnesiac gunslinger has memories of his prior time as a pacifist preacher, before his family was attacked and killed. Eventually he remembers his life before that, including his time as a merciless killer which caused his guilt to turn him to pacifism. He also remembers his life in the pre-apocalyptic world, from where he and others were thrust forward in time; he recalls that this is one of countless parallel Earths, on one of which a version of a former companion has harnessed magical power to become a superbeing by absorbing the life force of everything on the planet and now intends to continue the process on the parallel worlds, unless the hero and all the friends and comrades he has made can stop him.