For months, The Real Sabu, as he called himself on Twitter, boasted, cursed and egged on his followers to take part in computer attacks against private companies and government agencies worldwide. “Don’t give in to these people,” he wrote on Monday, ridiculing “cowards” in the federal government. “Fight back. Stay strong.” It turns out that Sabu had become an informant for federal law enforcement authorities. On Tuesday, in what could be one of the biggest breakthroughs in the government crackdown on a loose, large confederation of politically inspired “hacktivists,” he was unmasked and revealed to have helped the authorities catch...