Strongly derogatory term for a cruel, inhuman person. Often used in fictional or dramatic contexts to describe villains or monstrous characters.
He's said to be a man like a fiend.
He committed a brutal, inhuman act.
You fiend!
The culprit repeated his fiendish acts.
The public called him a brute.
畜生 is a milder expletive meaning 'beast' or 'damn it,' while 鬼畜 is a much stronger term implying demonic cruelty toward others.
人非人 is a formal, literary term for someone who has lost all humanity, whereas 鬼畜 is more common and stresses both demonic and animalistic cruelty.
鬼畜 literally means 'devil and beast' and is used for someone who commits inhuman acts, while 外道 in this sense is a broader insult for a cruel or repugnant person.
Compound of 鬼 (oni, 'demon') and 畜 (livestock, beast), literally 'demon beast'. The exact historical derivation is uncertain, but the combination conveys extreme cruelty and inhumanity.