noun
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.
その殺人鬼は十人以上の犠牲者を出した。
That homicidal maniac claimed more than ten victims.
彼は冷酷な殺人鬼として知られている。
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.
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.