Albert Barnes 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. Not manifesting the spirit that Cain showed. His was a most remarkable and striking instance of a lack of love for a brother, and the case was well suited to illustrate the propriety of the duty that the apostle is urging. 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 eager to guard those to whom he wrote against the indulgence of any feelings that were the opposite of love, and from anything like envy toward more highly favored fellow believers, by showing what this would lead to if fully acted out, as in the case of Cain.

A large part of the crimes of the earth have been caused, as in the murder of Abel, by the lack of brotherly love. Nothing but love would be necessary to put an end to the crimes and, consequently, to a large part of the misery of the world.