Former Baltimore City homicide detective Kelvin Sewell has seen it all. Gang members burned alive; a baby unceremoniously stuffed into the ground by its own mother; a sex offender who killed a child in a delusional jealous rage. The constant grind of bearing witness to violent death has given Sewell an unprecedented perspective into the minds of killers. He sat in the Baltimore Police Department's interview room with 14-year-old Devon Richardson as the teen tried to explain why he shot a woman he didn't know in the back of the head. He watched the father of 17-year-old Nicole Edmonds cry over...