Jamieson, Fausset and Brown on 1 John 3:15

15. hateth--equivalent to "loveth not" (1Jo 3:14); there is no medium between the two. "Love and hatred, like light and darkness, life and death, necessarily replace, as well as necessarily exclude, one another" [Alford].

is a murderer--because indulging in that passion, which, if followed out to its natural consequences, would make him one. "Whereas, 1Jo 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 to be dead.

hath--Such a one still "abideth in death." It is not his future state, but his present, which is referred to. He who hates (that is, loveth not) his brother (1Jo 3:14), cannot in this his present state have eternal life abiding in him.