The vast majority of killers prey on their victims alone and undercover. But in a tiny number of cases, two people are drawn together to form a deadly duo. Some of the names are as famous as any criminals ? Brady and Hindley, Bonnie and Clyde ? others you will have never heard of. But when they do team up, killer couples are usually even more brutal and sadistic than those who murder alone. How on earth do two people come to share an appetite for rape, torture and murder - and why would they risk life and liberty to share that secret with a second person?

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