Why should Christians love one another? Because it is a just and righteous requirement. As the apostle John further states: “Beloved ones, if this is how God loved us”​—by sending forth “his Son as a propitiatory sacrifice for our sins”—​“then we are ourselves under obligation to love one another.” And it is also the course of wisdom for Christians to love one another, for “love builds up”; it builds up both the one loving and the one being loved.​—1 John 4:10, 11; 1 Cor. 8:1; Acts 20:35.

“Everyone who hates his brother is a manslayer, and you know that no manslayer has everlasting life remaining in him.” Cain, the first son of Adam, gives us a warning example in this regard. Hate caused him to ignore God’s reproof and deliberately to murder his brother Abel.​—1 John 3:15.

No doubt hopes ran high in Adam and Eve upon the birth of their first son Cain. This seems to be indicated by Eve’s words: “I have acquired a man with the aid of Jehovah.” (Gen. 4:1) It could well be that Cain received special consideration as the firstborn and that he let it go to his head. When his brother Abel came along, Cain no doubt looked down upon him. He was a proud man.

All this came out when Cain and Abel made offerings to Jehovah God. Cain brought fruits and vegetables, whereas Abel “brought some firstlings of his flock, even their fatty pieces. Now while Jehovah was looking with favor upon Abel and his offering, he did not look with any favor upon Cain and upon his offering.” (Gen. 4:3-5) Why? Because Abel had the right heart condition, offered the right sacrifice and offered it in faith, but in all these respects Cain was lacking.​—Heb. 11:4; 1 John 3:12.

To see Abel, whom he considered inferior to himself, being preferred by Jehovah God was just too much for Cain. A murderous hate filled his soul: “Cain grew hot with great anger, and his countenance began to fall.” Seeing Cain’s heart condition, God extended a helping hand to him by reproving Cain: “Why are you hot with anger and why has your countenance fallen? If you turn to doing good,” that is, if you exercise faith and offer the right kind of sacrifice, “will there not be an exaltation? But if you do not turn to doing good, there is sin crouching at the entrance, and for you is its craving; and will you, for your part, get the mastery over it?”​—Gen. 4:5-7.

Here Jehovah God, in his love and long-suffering, was warning proud, envious Cain about his dangerous heart attitude and telling him that he too could have Jehovah’s favor if he would humble himself by copying Abel’s example. It was an animal sacrifice that was fitting as it involved the shedding of blood, and blood needed to be shed to reconcile man with God.​—Heb. 9:22.

But Cain was not listening to Jehovah God. His pride and envious hatred had not only hardened his heart against his brother but even made him deaf to reproof by Jehovah God himself. “After that Cain said to Abel his brother: ‘Let us go over into the field.’ So it came about that while they were in the field Cain proceeded to assault Abel his brother and kill him.” Evidently Abel was so guileless that he did not at all note his brother’s envious hatred and so went along, wholly unsuspecting any foul play.​—Gen. 4:8.

Cain killed his brother in cold blood. It was no impulsive deed, done on the spur of the moment in the heat of passion. It was premeditated murder. Jehovah God had warned him, and he had chosen to ignore that warning. He invited his brother to go into the field away from the rest of their family for the purpose of killing him. Then when Jehovah asked him: “Where is Abel your brother?” Cain hypocritically and lyingly replied: “I do not know. Am I my brother’s guardian?” (Gen. 4:9) For his hateful murder Cain lost out on any hope of everlasting life, even as indicated by the words of John.​—1 John 3:12.