Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.
Hateth - "loveth not," 1 John 3:14: there is no medium. 'Love and hatred, like light and darkness, life and death, necessarily replace, as well as exclude, one another' (Alford).
Is a murderer - because indulging that passion, which, if followed to its natural consequences, would make him one. '1 John 3:16 desires us to lay down our lives for the brethren; duels require one (awful to say!) to risk his own life, rather than not deprive another of life' (Bengel). God regards the inward disposition as tantamount to the outward act which would flow from it. Whomsoever one hates, one wishes dead.
Hath - such a one still "abideth in death." Not his future state, but his present is refuted to. He who hates (i:e., loveth not) his brother (1 John 3:14) cannot, in his present state, have eternal life abiding in him.