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Earlier this month, Tunisian opposition leader Chokri Belaid was shot dead outside his home. Belaid’s death has shaken Tunisia, but it also illuminates larger trends in the post-revolution Arab world.

Belaid was an intrepid critic of the authoritarian ancien regime of Zine Abidine Ben Ali, which ruled Tunisia for nearly a quarter century until it was felled by a revolution in January 2011. He was also a leader of the left-wing Democratic Patriots’ Movement and emerged as a fierce foe of Ennahda, the moderate Islamist party that formed an uneasy coalition with two secular parties. But Belaid was unimpressed by that step, or by Ennahda’s promises to pursue moderate Islam and pluralism. He questioned the party’s sincerity about democracy and tolerance and also...