One of Chairman Mao’s favorite words was “contradictions,” and today’s China is riddled with them: rule by a party that is nominally Communist, but embraces consumerism and welcomes entrepreneurs into its ranks; widespread unease about the environmental, social and even moral consequences of growth; deep insecurity in the ranks of a party that outwardly brims with confidence. The dark side of the Chinese dream — the negative fantasy that haunts China’s psyche — explains why Mr. Xi, the strongest Chinese leader since Deng, is so skittish, so ready to jump at shadows.