The far-left radical movement known as Antifa has been getting its own taste of cancel culture. For some people who ascribe to the Antifa ideology, protests and petty vandalism have gotten old — with groups now turning to ghastly acts of violence. Experts who track domestic terrorism and extremist movements aren’t surprised: Once the face of a grassroots leftwing insurgency, Antifa has lost its spark. But experts warn that could be a dangerous thing, as the vacuum has given way to even more fringe and violent offshoots that are coalescing in the Marxist ether. And we’re only beginning to see...