What drives people to kill? Science has spent more than 50 years trying to get inside the mind of murderers to find in what ways they are different to the rest of us and also the ways in which they are disturbingly similar. Michael Mosley opens up the Horizon casebook to discover how scientists have used extraordinary techniques and experiments to see what goes on inside the brain of a murderer. He shows how they have created a surprisingly comprehensive picture of the different factors that can drive someone to kill. And he asks the ultimate question will we one day be able to intervene to prevent someone becoming a killer?