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.

The story of Cain is recorded in Genesis 4:1ff, where Cain's wickedness (which long preceded the murder of Abel) at last issued in his offering being rejected by God. In the ensuing hatred of Abel, Cain killed his brother. It is an important point to remember why God rejected Cain's offering. Stott has a remarkably clear word on this:

If Cain had done well, his offering would have been accepted (Genesis 4:7). According to Hebrews 11:4, it was by "faith" that Abel offered a more excellent sacrifice than Cain ... we may assume that God had revealed his will to the two brothers ... By faith Abel obeyed ... Cain was willfully disobedient.[28]

Stott's deductions in this are so obviously true that one may only wonder about those who consciously try to make allowances for Cain.

Cain was of the evil one ... It is a mistake to suppose that God punished Cain merely for making a mistake in the worship; this reveals that Cain was controlled by evil principles. "It is inferred here that even before Cain slew Abel, there was something in the actions of the brothers that revealed their difference."[29] The New Testament reveals that Abel was righteous and that Cain's works were evil, as this very verse flatly declares.

And slew his brother ... This sheds further light upon what is recorded in Genesis 4, where it is recorded merely that Cain rose up and slew his brother. The word John used in this place properly means: "slaughtered," "butchered," "by cutting the throat ("jugulare") like an ox in the shambles."[30]

And wherefore slew he him ... ? It was not for any offense of Abel's against his brother, but simply and only because, "Cain's works were evil, and his brother's righteous." Thus quite early in human history the hatred of darkness against the light was revealed. Cain was the archtype of the world's eternal opposition to truth and righteousness. Roberts was of the opinion that John's choice of Cain as his example of evil could very possibly have been due to the fact that the odious heresy of the Cainites (which flourished a little later) might already have made its appearance at the time he wrote.[31]

The heroes worshipped by this monstrous system were Cain, Korah, the Sodomites and Judas Iscariot. They advocated such nonsense by means of a "Gospel of Judas." ... They taught that men could not be saved until they had passed through every kind of experience, even the most vile, claiming that an angel attended their orgies and urged them on to incur pollution. Out of their debaucheries, they claimed to have "perfect knowledge," and did not shrink to rush into such actions as it is unlawful even to name?[32]

[28] John R. W. Stott, Tyndale New Testament Commentaries, Vol. 19 (Grand Rapids, Michigan: Wm. B. Eerdmans Publishing Company, 1964), p. 140.

[29] J. W. Roberts, op. cit., p. 89.

[30] David Smith, op. cit., p. 185.

[31] J. W. Roberts, op. cit., p. 89.

[32] Irenaeus, Against Heresies I, 31 in The Ante-Nicene Fathers (Grand Rapids, Michigan: Wm. B. Eerdmans Publishing Company, n.d.), Vol. I, p. 358.