In cases of accidental murder, God set up six cities or refuge. If one who had committed murder, one could run to one of those cities to seek shelter. Judges in the city would hear his or her case and then rule if it was accidental or self defense. If it was judged either of the two, they could stay. Those who had not killed in self defense or accidental were not permitted to stay in these cities.
Abner, after killing Asahel, went to one of those cities for refuge. Unfortunately, he stopped at the gate and was taken aside by Joab and Abishai and they killed him.
David, said when he heard that Abner was slain, 'Died Abner as a fool dieth.
God allowed even a murderer who had accidentally killed to be put to death by the 'avenger of blood' if that one did not make it safely into one of those six cities. The avenger of blood was always a family member.
Now, many of you know I have a convicted killer in the family. My foster brother, back in the early 80's shot and killed a woman for 50 dollars to support a drug habit. Instead of being given the death penalty by the judge, he was given life pluse 54 years. Joe will never get out of prison.
While there, Joe was convicted of another murder behind bars. He was then sentenced to death and was moved to death row. Had Joe been put on death row after the first murder, many from both families would have not had their hearts torn so when he was sentenced for the second murder.
This month, I had a cousin shot and killed by her best friend. My cousin was 6 months pregnant and the baby was delivered by Caesarian section. Baby Hailey has had several heart attacks and her kidneys have shut down since the tragic event of her mother's death.
I could not seek the death penalty of the girl who shot my cousin, but I cannot blame any of my family who decided that they should be an 'avenger of blood' and seek the death penalty. God allows that, I believe.
Punishment for sin will be given in heaven, but punishment for crime must be dealt with here on earth. This is why God set up authorties such as judges and magistrates.
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One more thing, David wrote in the Book of the Psalms, 'Forever, O Lord, Thy Word is settled in heaven.' And Jesus said in the New Testament, 'Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.'