I've read with interest and have a few comments I hope will help.
Concerning the gravity of hating a brother (Gr. adelphos, one of the same womb or counted as fellow child of God), Jesus said that "is" murder, being a "manslayer" as though running a sword through them. John wrote similarly.
1 John 2:9 "He that saith he is in the light, and hateth his brother, is in darkness even until now."
1 John 2:11 "But he that hateth his brother is in darkness, and walketh in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes."
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 4:20 "If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?"
The object lesson is what is the present state? A person might be a former murderer, which class could contain all of us to some degree. Scriptures don't distinguish between levels of murderer. A person might currently be trapped into a murder lifestyle. Another might go to his grave as a habitual murderer. God is interested in the current status of the heart. If you have a murderous heart of any degree, repent. If a manslayer, confess it, submit to the laws of man and of God, and never do it again. A murderER is one who chooses to retain the sin of murder. Many people are Christians today that have murderED in some way in the past, but certainly not ongoing. We can't be to judgmental over just the sin of murder. Rev. 21:8 "But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death." Shed all those sins, don't be counted as a sinnER, but be holy as He is holy, a saint of God. Have you told a lie lately? It will command the same attention as any of those other sins. We've all sinnED, but shall no more be slaves to sin, Christians!
Who can say they had no sin in them worthy of eternal death before being born again? Please don't deceive yourself into saying "Me". Just being a habitual hater of a family member or just one neighbor makes one a murderer in God's eyes because that person lives in darkness. But He saves from such as that. Once in Christ the goal is never to allow any sin to take root, to become a habit. We all are tempted in our mind saved or not, without a problem unless temptation is allowed to grow into lust or covetousness. It is indeed likely that if anyone harbors a temptation too long, applies the imagination to it, then lets that become lust, that lust can drive a person's ego until death. Is that the sort of mindset God administers in His Kingdom? Do any of us really believe God wants people with murderous thoughts to live eternally with Him?
But if a person lets all that go to the end of actually slaying a brother, we know the love of God is not there. There remains no wondering then. But if God cannot forgive that and save a soul guilty of hating a brother, then who can be saved? None. No murderer has eternal life. But the key is murderer formerly, but no more, having repented unto forgiveness. But if always a murderer, by not confessing it, or by continually slaying, then that person remains unforgiven and without hope. The scriptures are quite clear. No unrepentant unrighteous person will have eternal life, regardless of how much God might choose anyone. He will not choose unrepentant sinners. Instead, God chose the plan that by faith in Christ any sinner can be saved and break free of the curse of sin, the law, and slavery to behave contrary to the holy righteousness of Christ.
There was no law against murder when Cain slew Abel. God is not guilty of giving men the concept of a sin not committed, but is on record dealing with every sin after committed. God immediately put a curse on anyone that had ideas of killing Cain in revenge. He didn't put the threat of death on that sin until a few thousands years later, announced by Moses. Once God prohibits an act He means don't do it. He dealt with the physical act of slaying a person on a personal basis, such as out of hatred. He also defined and strongly condemned adultery and homosexuality, more abominable sins God said are worthy of death. Jesus went deeper to say such sins are already accounted just for dwelling on them to the point of lusting, unrepentance for which assures eternal death as well as continuing in the curses of the law. It isn't the sins themselves that condemn, but the darkness one dwells in. The sins are symptoms of spiritual heart disease. None of that is beyond healing by God through Jesus and true repentance. The one sin that can't be forgiven, indicating a most unholy wickedness, is ascribing an act of the Holy Spirit to Satan while fully knowing better.