2 Samuel 3:30

And the cause why Joab and Abishai slew Abner was that Abner had slain their brother Asahel at Gibeon in battle.

2 Samuel 2:19-23

and followed after Abner and turned neither to the righthand nor to the left from Abner.

Proverbs 28:17

He that by violence sheddeth any man's blood, shall be a renagade unto his grave; and no man shall be able to succor him.

Acts 28:4

When the men of the country saw the worm hang on his hand, they said among themselves, "This man must needs be a murderer: Whom, though he have escaped the sea, yet vengeance suffereth not 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
He that by violence sheddeth any man's blood, shall be a renagade unto his grave; and no man shall be able to succor him.
Acts 28:4
When the men of the country saw the worm hang on his hand, they said among themselves, "This man must needs be a murderer: Whom, though he have escaped the sea, yet vengeance suffereth not to live."

Because

2 Samuel 2:19
and followed after Abner and turned neither to the righthand nor to the left from Abner.