Wendy Rogers - Credit: AP Before he went on a racist rampage in a Buffalo grocery store on Saturday killing 10 people, Payton Gendron is believed to have written a hate-filled screed promoting the conspiracy theory that white people are facing ethnic, cultural and racial displacement by immigrants — a.k.a., a “white genocide.” It is an extremist position promoted widely on the right, including by others who have carried out deadly attacks in places like El Paso and Pittsburgh. Among the “deplorable” set — those on the alt-right for whom this “ great replacement theory ” has true cultural currency...