The second great commandment and the one Jesus gives us is “that we should love one another.” (1 John 3:11) “Whoever does not love abides in death” rather than in life. (1 John 3:14) And if we hate one another, then we are become as murderers, “and you know no murderer has eternal life abiding in him.” (1 John 3:15) Simply, the author is telling us that hatred leads to eternal death while love leads to eternal life.
The first murderer, Cain, murders his own brother in a fit of jealous rage. “And why did he murder [Abel]? Because his own deeds were evil and his brother’s righteous.” (1 John 3:12) And this is the judgment: the Light has come into the world, and people love the darkness rather than the Light because their works are evil. For everyone who does wicked things hates the Light and does not come to the Light, lest his works should be exposed. But whoever does what is true comes to the Light, so that it may be clearly seen that his works are carried out in God.” (John 3:19-21)
The author of 1 John actually warns us that “we should not be like Cain.” (1 John 3:12) If we become like Cain, then we are “not of God” for the “one who does not love his brother” can not be “born of God” (1 John 3:10,9)
Yet, “whenever our heart condemns us, God is greater than our heart, and He knows everything.” (1 John 3:20) “Little children, let us not love in word or talk, but in deed and in truth.” (1 John 3:18) Let us come to the Light so that our “works are carried out in God.” (John 3:21)