Adam Clarke on 1 John 3:15

Whosoever hateth his brother is a murderer - He has the same principle in him which was in Cain, and it may lead to the same consequences.

No murderer hath eternal life - Eternal life springs from an indwelling God; and God cannot dwell in the heart where hatred and malice dwell. This text has been quoted to prove that no murderer can be saved. This is not said in the text; and there have been many instances of persons who have been guilty of murder having had deep and genuine repentance, and who doubtless found mercy from his hands who prayed for his murderers, Father, forgive them; for they know not what they do! It is, however, an awful text for the consideration of those who shed human blood on frivolous pretences, or in those wars which have their origin in the worst passions of the human heart.