Mark Pedersen grew up in a midwestern town poisoned by lead. Disease and death were everywhere. In search of meaning, he started treating chronically ill kids using medical marijuana—until one of them died. Then the law came after him.

It was a brisk summer afternoon in Lakewood, Colorado, August 24, 2016. Mark Pedersen was on the back deck when he noticed the commotion. First he glimpsed flashes of red pulsing across the front of …