The Greek compound anthrōpoktonos means murderer or man-slayer, combining anthrōpos (human being) and kteinō (to kill). Occurring 3 times in the NT, it is used twice by Jesus to describe Satan (John 8:44) and once by John to describe the state of those who hate their brothers (1 John 3:15).
Jesus' statement that Satan 'was a murderer from the beginning' (John 8:44) traces the origin of death to the devil's deception in Eden — his lie led to the spiritual and physical death of humanity. Anthrōpoktonos thus names the deepest nature of evil: it is anti-life, anti-human, and anti-God. John's use in 1 John 3:15 is shocking: 'Anyone who hates a brother or sister is a murderer.' Hatred in the heart is classified with the same word used for Satan. This is the logic of the Sermon on the Mount: the heart matters, and murderous hatred is already murder before the act.