Halloween is the one night a year we tell children to knock on strangers’ doors, and tell strangers to smile and hand over sweets. “Treat or Trickturns that around. A small boy stands in a leafless, moonlit forest and offers you a wrapped candy in his open palm. He looks up from under his brows, almost smiling, in a T-shirt with a jack-o’-lantern on it. His other arm hangs behind his body...