Albert Barnes on 1 John 2:11

But he that hateth his brother. The word here used would, in this connexion, include both the mere absence of love, and positive hatred. It is designed to include the whole of that state of mind where there is not love for the brethren.

Is in darkness. 1Jn 2:9.

And walketh in darkness. He is like one who walks in the dark, and who sees no object distinctly. Jn 12:35. And knoweth not whither he goeth. Like one in the dark. He wanders about not knowing what direction he shall take, or where the course which he is on will lead. The general meaning is, that he is ignorant of the whole nature of religion; or, in other words, love to the brethren is a central virtue in religion, and when a man has not that, his mind is entirely clouded on the whole subject, and he shows that he knows nothing of its nature. There is no virtue that is designed to be made more prominent in Christianity; and there is none that will throw its influence farther over a man's life.

(a) "walketh in darkness" Prov 4:15, Jn 12:35