He that saith he is in the light, and hateth his brother, is in darkness even until now.
There is no mean between light and darkness, love and hatred, life and death, God and the world: wherever spiritual life is, however weak, there darkness and death up longer reign, love supplants hatred. Luke 9:50 holds good: wherever life is not, there death, darkness, the flesh, the world, and hatred, however hidden from man's observation, prevail. Luke 11:23 holds good: 'where love is not, there hatred is; for the heart cannot remain a void' (Bengel).
In the light - as his proper element.
His brother - his neighbour, especially the Christian brotherhood. The title brother is a reason why love should be exercised.
Even until now - notwithstanding that 'the true light already has begun to shine' (1 John 2:8).