Contract killers are employed by organized crime groups to eliminate their rivals. Assassins receive no gratification from their killings and require no cooling off period between murders. Killing is just business. read more
The cooling off period of serial killers and their thrill of killing for pleasure is very disturbing, but we view these "professional assassins" as just hit men getting their money from their employers. read more
There are different terms for killers and murderers who kill in different time periods or manners (i.e spree killer for many people in a single instance). I think your confusion arises from nomenclature. We as a society tend to prefer simple labels to complete ones. A hitman can also be a spree or serial killer. read more