John Calvin Commentary 1 John 3:15

"Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him." — 1 John 3:15 (ASV)

Is a murderer — To stimulate us further to love, he shows how detestable hatred is before God. There is no one who does not dread a murderer; indeed, we all detest the very name. But the Apostle declares that all who hate their brothers are murderers.

He could have said nothing more atrocious; nor is this statement hyperbolical, for we wish the one whom we hate to perish. It does not matter if a person keeps their hands from doing harm, for the very desire to do harm, as well as the attempt, is condemned before God. Indeed, even when we do not ourselves seek to cause injury, if we wish for evil to happen to our brother from someone else, we are murderers.

Then the Apostle defines the matter simply as it is when he attributes murder to hatred. This proves human folly: though people detest the name, they still take no account of the crime itself. Why is this? It is because outward appearances occupy our thoughts, but the inward feeling is brought to account before God. Therefore, let no one minimize such a grievous evil any longer. Let us learn to refer our judgments to the tribunal of God.