2 Samuel 3:30

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

2 Samuel 2:19-23

Asahel pursued Abner and did not turn to the right or to the left as he followed him.

Proverbs 28:17


A man who is burdened with the guilt of human blood (murder)
Will be a fugitive until death; let no one support him or give him refuge.

Acts 28:4

When the natives saw the creature hanging from his hand, they began saying to one another, “Undoubtedly this man is a murderer, and though he has been saved from the sea, Justice [the avenging goddess] has not permitted him to live.”

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

A man who is burdened with the guilt of human blood (murder)
Will be a fugitive until death; let no one support him or give him refuge.
Acts 28:4
When the natives saw the creature hanging from his hand, they began saying to one another, “Undoubtedly this man is a murderer, and though he has been saved from the sea, Justice [the avenging goddess] has not permitted him to live.”

Because

2 Samuel 2:19
Asahel pursued Abner and did not turn to the right or to the left as he followed him.