Adam Clarke's Commentary on the Bible

Verse 12 (1 John 3:12)

Not as Cain - Men should not act to each other as Cain did to his brother Abel. He murdered him because he was better than himself. But who was Cain? Εκ του πονηρου ην , he was of the devil. And who are they who, through pride, lust of power, ambition, gain, etc., murder each other in wars and political contentions? Εκ του πονηρου εισι . To attempt to justify the principle, and excuse the instigators, authors, abettors, etc., of such wars, is as vain as it is wicked. They are opposed to the nature of God, and to that message which he has sent to man from the beginning: Love one another. Love your enemies. Surely this does not mean, Blow out their brains, or, Cut their throats. O, how much of the spirit, temper, and letter of the Gospel have the nations of the world, and particularly the nations of Europe, to learn!

And wherefore slew he him? - What could induce a brother to imbrue his hands in a brother's blood? Why, his brother was righteous, and he was wicked; and the seed of the wicked one which was in him induced him to destroy his brother, because the seed of God - the Divine nature, was found in him.

- Adam Clarke's Commentary on the Bible