1 John 3:12

“Not as Cain, &c.] 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 f37, that “he was of the side of the serpent (the old serpent the devil); and as the way of the serpent is to slay and to kill, so Cain immediately became a murderer.” And again, “because…”

“who--not in the Greek. of that wicked one--Translate, "evil one," to accord with "Because his own works were evil." Compare Jo1 3:8, "of the devil," in contrast to "of God," Jo1 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.”

“Not as Cain - Not manifesting the spirit which Cain did. His was a most remarkable and striking instance of a want of love to a brother, and the case was well adapted to illustrate the propriety of the duty which the apostle is enjoining. See Genesis 4:4-8. Who was of that wicked one - Of the devil; that is, he was under his influence, and acted from his instigation. And wherefore slew he him? - Because his own works were evil, and his brother’s righteous.” He acted under the influence of envy. He was dissatisfied that his own offering was not accepted, and that his brother’s was. The apostle seems desirous to guard those to whom he wrote against the indulgence of any feelings that were the…”

“Who was of the wicked one - Who showed he was a child of the devil by killing his brother. And wherefore slew he him - For any fault? No, but just the reverse; for his goodness. 1 John 3:13”

“3:12-13 Cain had been doing what was evil: Cain was jealous that Abel received approval from God; this jealousy led to murder (Gen 4:8). Hatred is judged, just as the outward act that results from it (see Matt 5:21-22).”

“as: Gen 4:4-15, Gen 4:25; Heb 11:4; Jde 1:11 of: Jo1 3:8, Jo1 2:13, Jo1 2:14; Mat 13:19, Mat 13:38 And: Sa1 18:14, Sa1 18:15, Sa1 19:4, Sa1 19:5, Sa1 22:14-16; Psa 37:12; Pro 29:27; Mat 27:23; Joh 10:32, Joh 15:19-25, Joh 18:38-40; Act 7:52; Th1 2:14; Pe1 4:4; Rev 17:6 and his: Mat 23:35; Luk 11:51; Heb 11:4, Heb 12:24 1 John 3:13”