His method was patient and financial before it turned fatal — forged documents, false inheritances, the slow capture of trust — making the violence that followed harder to detect and easier to conceal. Operating in the dense commercial center of Milan, he used a basement on a narrow lane to hide at least four victims, their bodies discovered only after investigators followed a paper trail of fraudulent power of attorney. His case became notable not only for the crimes themselves but for what came after: his execution was the last civilian death sentence carried out in Milan before the abolition of capital punishment, and his remains were claimed by the emerging science of criminology, with Cesare Lombroso citing him as evidence for theories of innate criminal character.

Cook's 22-day rampage across the American Southwest drew national attention not only for its body count but for its randomness — victims were strangers who happened to offer a ride or cross his path at the wrong moment. The murder of the Mosser family, including three young children, marked the most concentrated act of violence in a spree that also encompassed kidnapping, robbery, and the killing of a traveling salesman. His capture came not through American law enforcement but through the initiative of a Mexican police chief who recognized him and physically disarmed him. Cook was ultimately executed in California's gas chamber for Dewey's murder, having already been sentenced to 300 years for the federal kidnapping charges.