A stew of hypertoxic rhetoric has surged through social media and American discourse after the killing of conservative activist Charlie Kirk, inflaming a political environment that was already deeply polarized. The most concerning messages, experts told NBC News, are proclamations from far-right activists, Republican politicians and conservative influencers about a coming civil war and the need for retribution or payback against the left for Kirk’s killing. The phrase “civil war” has spiked on social media and in Google searches. “The Left is the party of murder,” Elon Musk posted on X to his 226 million followers, shortly after the shooting...