Jamieson, Fausset and Brown on 1 John 3:12

12. who--not in the Greek.

of that wicked one--Translate, "evil one," to accord with "Because his own works were evil." Compare 1Jo 3:8, "of the devil," in contrast to "of God," 1Jo 3:10.

slew he him? Because his own works were evil, and his brother's righteous--through envy and hatred of his brother's piety, owing to which God accepted Abel's, but rejected Cain's offering. Enmity from the first existed between the seed of the woman and the seed of the serpent.