“But he who hates his brother is in darkness and walks in darkness, and does not know where he is going, because the darkness has blinded his eyes.”
and does not know where he is going,
The word “going” means to move away from a reference point. The believer who harbors hate in his heart will lose his sense of spiritual direction. He wanders aimlessly in the dark in peril of a great fall. Malice causes him to lose his sense of spiritual judgment. He no longer has a spiritual reference point.
because the darkness has blinded his eyes
Unconfessed sin causes loss of intimate fellowship with God (1:5-2:2). Hatred is a symptom of spiritual darkness. Only the Holy Spirit can give him spiritual light when he confesses his sin.
Hatred prevents spiritual growth.
Hatred thwarts spiritual growth. God wants us to love other believers as He loves them. He wants us to love others the way He loves us. God never shows malice toward us.
If we hate our brother or sister in Christ, a self-chosen sphere of darkness blinds us. It does not merely impair our vision but constitutionally blinds us to fellowship with the Lord. We then stumble all over the place spiritually. Darkness creates its own momentum of vindictiveness and hatred.
Light shines on our path when we walk in fellowship with the Lord. We reflect the light of the glory of Christ who is the image of God. Darkness makes it utterly impossible to enter the sphere of glorifying Christ for we have no orientation to the God of light.