On 13 August, three states are set to carry out executions by lethal injection. It’s the first time since 2010 that three people will be put to death on the same day in the US. In Tennessee, the execution of Darrell Hines is in the spotlight, because the last lethal injection in the state was botched. Ed Pilkington speaks to Kai Wright about the doctor at the center of that botched execution, whose identity was revealed inadvertently last year. Ed explains how his reporting has lifted the lid on a role that is usually hidden away from public view, and what it tells us about the American death penalty today