TSK · Numbers 35:19

And they must stand as your towns of refuge from the avenger in order that the killer may not die until he has stood trial before the community.

then the community must judge between the slayer and the avenger of blood according to these decisions.

Otherwise the blood avenger will chase after the killer in the heat of his anger, eventually overtake him, and kill him, though this is not a capital case since he did not hate him at the time of the accident.

Anyone who accidentally kills someone can escape there; these cities will be a place of asylum from the avenger of blood.

He said,“They were my brothers, the sons of my mother. I swear, as surely as the LORD is alive, if you had let them live, I would not kill you.”

So Joab and his brother Abishai killed Abner, because he had killed their brother Asahel in Gibeon during the battle.