News Wednesday afternoon that an armed gunman had entered the cable TV headquarters of Discovery Communications in Silver Spring, Maryland and begun taking hostages alarmed people throughout the Washington, D.C. area and around the country. As law enforcement officials negotiated with the suspect, posts on social media outlets inevitably began arguing over the ideological motivations of the hostage-taker, James J. Lee. Conservatives were quick to point out the suspect's radical environmentalist manifesto, while left-leaning sources disclaimed any connection. News a few hours later that the suspect had been shot and killed by police spawned a round of smug black humor,...