1 John 3:14-15
14 We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death.
15 Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.
The Grace New Testament Commentary - 1 John 3:14-15
3:14 The emphatic we no doubt refers to the apostles themselves. In contrast to "the world," the apostles love their fellow Christians. Indeed, John declares, we [apostles] know that we have passed from death to life, because we love the brethren. This is more than an assertion that they love their fellow Christians. It is also a claim to a certain quality of experience. The apostles are able to recognize their experience of love as an experience of life rather than death.
The words passed from death to life do not mean the apostles were sure of their eternal salvation because they loved the brethren. There is no reason why this should be true for them or any other Christian. Assurance of salvation is based on the testimony of God (see 1 John 5:9-13). Instead, in a perfectly normal use of the word know John declares that he and his fellow apostles experience their passage from death to life through loving their Christian brothers. The implication is that the passage from death to life, which occurs at the point of salvation (John 5:24), can be experientially known and appreciated through Christian love.
By contrast he who does not love his brother abides in death. There is no way a Christian who fails to love his brother can have the immediate, experiential knowledge of life John has just spoken about. On the contrary such a person abides ("dwells") in death. If love is an experience of "life," John is saying, hatred of one's Christian brother is an experience of death (cf. Rom 7:9-10).
There is thus no reasonable objection to the concept of a Christian "abiding" in death in the sense that he has lost touch with the experience of God's life. In sharp contrast with 1 John 2:9-11; 3:10, 12, there is no Greek word for his in the phrase his brother. The statement of v 14 can then apply not only to Christians who might hate a particular Christian brother, but also to anyone else who might hate such a brother. It makes no difference who is doing the hating. Hatred by a Christian is an experience in the realm of death.
3:15 Hatred of one's brother is also an experience of murder. The person who hates his Christian brother is really no different from Cain (cf. v 12), even though he may not commit the overt act of physically killing his brother. The spirit of hatred is that a brother wants "to be rid" of his brother and would not really care if he died.
John does not say (as the NIV paraphrases), "No murderer has eternal life in him." The NKJV (and NAS) better translate the Greek as "No murderer has eternal life abiding in him". The key is the concept of "abiding." Moreover, John's concept of abiding is always that it is a reciprocal relationship, even as Jesus said; "Abide in Me, and I in you" (John 15:4; see 1 John 2:27). Since Christ Himself is eternal life (cf. 1 John 5:20), to say that someone does not have eternal life abiding in him is equivalent to saying that he does not have Christ abiding in him.