Story, first published: Galaxy Science Fiction, Oct. 1954
An alien visitor, convinced that Homo sapiens is a "treacherous and unprincipled species" inhabits the body of the village idiot, increasing his psi powers. The idiot uses his gift to reform the town and to revenge himself with ruthless justice on those who had made his life miserable. Then, he decides to extend his "crusade" to the whole world.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.56