The assassination of Charlie Kirk presages a new, deadly stage in the disintegration of a fractious and highly polarized United States. While toxic rhetoric and threats are lobbed across cultural divides like hand grenades, sometimes spilling over into actual violence — including the murder of Minnesota House of Representatives Speaker Emerita Melissa Hortman and her husband and the two assassination attempts against Donald Trump — Kirk’s killing is a harbinger of full-scale social disintegration.