We know that we have passed out of death into life, because we love the brothers. Whoever does not love abides in death. Everyone who hates his brother is a murderer, and you know that no murderer has eternal life abiding in him. (1 John 3:14-15 ESV)
John is speaking to Christians about loving your fellow Christian. The use of the word "the" in front of brothers (KJV says "the brethern) indicates that he is talking about those in the family of God and not all people in general.
Christians are to have a special bond of love with one another that they do not have with those outside the family of God. However, we are supposed to love our neighbor as ourself.
One who claims to be a Christian yet hates another Christian, or for that matter anyone, has committed murder in his heart. Hate is not a Christian virtue. There is a place for righteous, controlled anger that does not revolve around our pride, but there is no place for vindictive hate in the Christian's life. This verse tells us that this person is not Christian...they abide in death and do not have eternal life in them.
This is serious stuff. I need to seriously examine my heart when I find myself expressing venom for another person, especially another Christian. When I see other Christians "bite and devour" one another with their words, where does that come from? Do I join in?
Examine yourselves, to see whether you are in the faith. Test yourselves. Or do you not realize this about yourselves, that Jesus Christ is in you?—unless indeed you fail to meet the test! (2 Corinthians 13:5 ESV)