What if a murderer didn't repent before they died?

If they believed in the Lord Jesus Christ, they go to heaven.  If they didn't believe, they go to hell.  Here is the truth: The Lord Jesus Christ is God. Nobody can be good enough to avoid hell. The truth is that death leads to immediate heaven or hell, depending only on whether the person believed on the Lord Jesus Christ, or not. The truth is that the Lord Jesus Christ loves you and wants to bless your life freely :) Most of all, Jesus wants you with Him forever, and not in hell.
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Believing on the Lord Jesus Christ means being forgiven all sins past and future and means going to heaven. The truth is that every belief except one will lead to eternal torment in the lake of fire for every person. Because nothing pays for our sins except the death and blood of Jesus, the sacrifice of Jesus that is already accomplished by Him . Jesus loves you! The truth is that the Lord Jesus Christ is God, and Jesus died on the cross to pay for all of our sins in full, and then Jesus was buried, and then Jesus resurrected from the dead. Nothing else pays for our sins, not works, not deeds, not religions. So the only way to heaven and to avoid hell, is by believing on Jesus for eternal life (John 6:47), without adding any of your own works (Romans 4:5). Believe in the Lord Jesus Christ to take you to heaven, and you will be in heaven, no matter what, guaranteed. That easy, thanks to Jesus! Tell Jesus that you thank Him that you will be with Him in heaven

Apparently all of us hated in our heart. We are all murderers. 

Matthew 5
21 “You have heard that it was said to the people long ago, ‘You shall not murder,[a] and anyone who murders will be subject to judgment.’ 22 But I tell you that anyone who is angry with a brother or sister[b][c] will be subject to judgment. Again, anyone who says to a brother or sister, ‘Raca,’[d] is answerable to the court. And anyone who says, ‘You fool!’ will be in danger of the fire of hell."

Romans 6:7 says: “For the one who has died has been acquitted from his sin.”

The Greek word di·kai·oʹo used here is often rendered “to declare righteous.” The context shows that Paul was discussing spirit-anointed Christians alive at that time. They had been baptized into Christ Jesus and had received the valid prospect of heavenly life. However, in order to be anointed with holy spirit and accepted as spirit-begotten sons of God, they had to die figuratively to their former course of life as imperfect humans and have their sins forgiven by God. Then they could have human perfection imputed to them. In discussing this with regard to anointed Christians, Paul was drawing on a fundamental truth. He knew that the penalty for Adam’s sin was death. (Ge 2:17) So Paul reasons that one who has died has been acquitted from sin because by means of his death, he has paid the full penalty for sin. At Ro 6:23, Paul says: “The wages sin pays is death.” So when a person has died, his sinful record no longer stands against him. And if it were not for Jesus’ sacrifice and God’s purpose to resurrect the person, he would never live again. Still, he would remain acquitted from sin, since God would not reexamine his case and then sentence him to further punishment.

He would still die, Duh !

What if you found out he was innocent after you executed him?

Then God will grant them justice and send them to eternal Hell.

Then 1 John 1:9 would not help them.

Superstiion has no rules so believe whatever you are comfortable with

Maybe they weren't indoctrinated? In any case repenting has no effect whatsoever. Once you are dead that's it for all of us. You too.