In Cold Blood is of course a true story. Perry and Dick in the story formed the design of robbing the Clutter family and further planned to leave no survivors. They spent some time planning the crime, then drove several hundred miles to follow through. The multiple murders they committed were thus done cooly, deliberately, and with multiple opportunities to reflect on the enormity of their crime. Throughout the book, they present no redeeming qualities: they write bad checks to survive; and later plan on murdering another victim who offers them a ride so that they can steal his car. They express no remorse until they reach the gallows, at which point Dick simply comments that there are "no hard feelings," and Perry indicates that it is too late to do so.
They thus represent evil in its most human form: those who kill others for personal gain with no remorse or reluctance.