Trophy Killing
Trophy Killing is a gruesome aspect of both serial killers and some war criminals in which the killer will collect parts of their victims as "trophies" - tales of trophy killing go back to the ancient head-hunters and scalping but was not considered a war crime until much later.
Tales of soldiers in Vietnam collecting skulls, fingers and even human teeth highlighted the dangers of soldiers becoming savage killers and although a crime many units passed a blind eye to the practice.
Another version of Trophy Killing is when a killer does not collect body parts but instead, has a tally of people he or she has killed - many fighter-pilots were said to have symbols on their aircraft signaling the number of enemy pilots they had killed.