Watch Those Who Kill Online. Those Who Kill is based on a popular Danish crime series format inspired by the bestselling work of author Elsebeth Egholm. The series centers on Catherine Jensen (Sevigny), a freshly minted and incredibly smart police detective who tracks down serial killers. Jensen enlists the help of Thomas Schaffer (James D’Arcy), a forensic psychiatrist, to help her get into the minds of serial killers, all the while luring Schaffer into her own personal investigation of her missing brother.