TSK · Numbers 35:19

The cities shall be to you for refuge from the avenger, that the manslayer not die, until he stands before the congregation for judgment.

then the congregation shall judge between the striker and the avenger of blood according to these ordinances;

lest the avenger of blood pursue the manslayer, while his heart is hot, and overtake him, because the way is long, and strike him mortally; whereas he was not worthy of death, inasmuch as he didn't hate him in time past.

that the manslayer who kills any person accidentally or unintentionally may flee there. They shall be to you for a refuge from the avenger of blood.

He said, "They were my brothers, the sons of my mother. As Yahweh lives, if you had saved them alive, I would not kill you."

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