2 Samuel 3:30

So Joab and Abishai his brother put Abner to death, because he had put to death their brother Asahel in the fight at Gibeon.

2 Samuel 2:19-23

Asahel went running after Abner, not turning to the right or to the left.

Proverbs 28:17

One who has been the cause of a man's death will go in flight to the underworld: let no man give him help.

Acts 28:4

And when the people saw it hanging on his hand, they said to one another, Without doubt this man has put someone to death, and though he has got safely away from the sea, God will not let him go on living.

Treasury of Scripture Knowledge

Summary

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

Because

Bible References

Slew abner

Proverbs 28:17
One who has been the cause of a man's death will go in flight to the underworld: let no man give him help.
Acts 28:4
And when the people saw it hanging on his hand, they said to one another, Without doubt this man has put someone to death, and though he has got safely away from the sea, God will not let him go on living.

Because

2 Samuel 2:19
Asahel went running after Abner, not turning to the right or to the left.

Basic English, produced by Mr C. K. Ogden of the Orthological Institute - public domain