Theophylact of Bulgaria
The Apostle confirms by example how great an evil it is to hate one's brother. See, he says, Cain hated his brother and killed him, although he was his own brother. Cain was doing the evil works of his father, the devil. Abel, practicing righteousness, was a son in relation to God. Here the devil is set against God, and evil works against good ones. Therefore Cain, being opposed to his brother, killed him. Such is the explanation according to the course of the discourse. But the Apostle John changed this order for a special reason. After mentioning the devil and saying that those who do evil become his children, it was natural for him to set forth as an example the one who, at the beginning of the world, hastened to become a son of the devil through evil deeds. Having said what was needed about Cain and the lot inherited by those who imitate him, he again returns to love.