John Gill 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)

Not as Cain
That is, let us not be like him, or do as he did, hate the brethren. The apostle illustrates brotherly love by its contrary, in the instance of Cain, who was the first instance and example of hatred of the brethren, and of fratricide, and a very detestable one, by which he would dissuade from so vile and abominable a practice:

[who] was of that wicked one ;
Satan, a child of his, an imitator of him, one that appeared to be under his influence, and to belong unto him. So the Jews say of Cain ^{F14 F15 F16} .

And slew his brother ;
see (Genesis 4:8) . ^F17

and wherefore slew he him ?
what was the cause and occasion of it? what moved him to it?

because his own works were evil, and his brother's righteous ;
or "his work", as the Ethiopic version reads: the sacrifice which he offered up, which, though it was not evil as to the matter and substance of it, yet was so, being offered with an evil mind, and with an hypocritical heart, and without faith in the sacrifice of Christ, and so was unacceptable to God; whereas, on the other hand, the sacrifice his brother brought was offered up in the faith of Christ, by which he obtained a testimony that he was righteous, and that the work he did was a righteous work, being done in faith, and so was acceptable to God; which Cain perceiving, was filled with envy, and this put him upon killing him. ^F18

In the Hebrew text in (Genesis 4:8) ; there is an extraordinary large pause, as if a discourse of this kind, which passes between the two brothers, was to be inserted. ^F19

so that the controversy was about grace and works, as now; and as then Cain hated his brother upon this account, so now carnal men hate and persecute the saints, because they will not allow their works to be the cause of justification and salvation: and from hence also it may be observed, that a work may be, as to the matter of it, good, and yet as to its circumstances, and the end and view of it, evil.

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FOOTNOTES:

• F14: Midrash Ruth in Zohar in Gen. fol. 42. 4.
• F15: Zohar in ib. fol. 43. 1.
• F16: Ib. fol. 41. 1, 2.
• F17: Targum Jon. in Gen. iv. 8. Pirke Eliezer, c. 21.
• F18: Targum Hieros. & Jon. in Gen. iv. 8.
• F19: Quod Det. Potior. p. 161.