bloodthirsty killer; cold-blooded murderer; homicidal maniac
Refers to a person who kills with extreme cruelty or without remorse, often used in crime reporting, fiction, and psychological descriptions.
That homicidal maniac claimed more than ten victims.
He is known as a cold-blooded murderer.
殺人犯 is a legal/criminal term for a person who has committed murder, without the connotation of bloodthirstiness.
serial killer
Compound of 殺人 (satsujin, 'murder') and 鬼 (ki, 'demon, ogre'), literally 'murder demon'. The exact historical derivation is uncertain, but the word evokes a demon-like killer.