A.T. Robertson Commentary 1 John 3:12

"not as Cain was of the evil one, and slew his brother. And wherefore slew he him? Because his works were evil, and his brother`s righteous." — 1 John 3:12 (ASV)

Of the evil one (εκ του πονηρου). Ablative case and the same for neuter and masculine singular, but verse 10 makes it clear that the reference is to the devil.

Slew (εσφαξεν). First aorist active indicative of σφαζω, old verb, to slay, to butcher, to cut the throat (Latin jugulare) like an ox in the shambles, in N.T. only here and Rev. (Revelation 5:6,9,12, etc.).

Wherefore? (χαριν τινοσ; ). "For the sake of what?" Post-positive preposition (Ephesians 3:1,14) except here. The interpretation of the act of Cain (Genesis 4:8ff.) is an addition to the narrative, but in accord with Heb 11:4. Jealousy led to murder.