"For this is the message that ye heard from the beginning, that we should love one another. Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother’s righteous. Marvel not, my brethren, if the world hate you. We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death. Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him. Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren. But whoso hath this world’s good, and seeth his brother have need, and shutteth up his bowels of compassion from him, how dwelleth the love of God in him? My little children, let us not love in word, neither in tongue; but in deed and in truth." (1 John 3:11-18)

Whe we compare this portion of scripture with account of Cain and Abel found in Genesis 4, we quickly see that John was inspired to tell us a little more about Cain's motive for killing his brother. 

It was just that Cain was jealous. He killed his brother partly because Abel's deeds were righteous. Cain hated righteousness and therefore killed a person who was doing righteous things. 

John gives a Biblically authentic, historical account of what wicked people do. They hate righteousness and those that are righteous. 

I believe John includes this so that the children of God won't be surprised when people hate them for doing good.

This should also encourage us to act righteously even more.  If we claim to be disciples of Christ, then our acts of love toward others should show it abundantly. We know that God loves us because He acted on that love and sent His Son to die for us. If we love the Lord, we should show it by sacrificing ourselves for others. 

This is the same lesson James teaches us when he says "Faith without works is dead, being alone." 

Our love of the brethren is one of the clearest evidences we have that we are the sons of God. Why would we then not act out that love all the more so that we can feel that assurance more abundantly?