William Burkitt on 1 John 3:15

Observe here, 1. The nature of the sin condemned, it is a secret sin of the heart, not an open sin of the life; he that hateth his brother, that is, in his heart, is a murderer, though he doth not smite him either with his tongue, or with his hand.

Learn hence, That sins of the heart are damning, as well as sins of the life; a man may be an adulterer in the sight of God, and yet never touch a woman, Matt 5:28; an idolator, and yet never bow his knee to an image, Eph 5:5, a murderer, and yet never hurt his brother; if he hates him in his heart, it is recorded murder in God's account. What need have we to put up David's prayer, Cleanse thou me from my secret sins? Ps 19:1

Observe, 2. The sad and deplorable condition of such as are guilty of this sin, namely, of murdering their brother by hatred in their hearts: He that hateth his brother, abideth in death, and hath not eternal life abiding in him, 1John 3:14,15; that is he hath no spiritual life, nothing of the life of grace abideth in him, which is the seed and principle, the original and beginning of life.

Note thence, That the life of grace in the heart of a regenerate person, is the beginning and first principle of a life of glory, whereof they cannot but be destitute who hate their brother in their hearts. So much hatred in a man, so much death; and so much want of love, so much want of life.