2 Samuel 3:5 Now the sons of Rimmon the Beerothite, Rechab and Baanah, set out, and about the heat of the day they came to the house of Ishbaal, while he was taking his noonday rest. 6 They came inside the house as though to take wheat, and they struck him in the stomach; then Rechab and his brother Baanah escaped. 7 Now they had come into the house while he was lying on his couch in his bedchamber; they attacked him, killed him, and beheaded him. Then they took his head and traveled by way of the Arabah all night long. 8 They brought the head of Ishbaal to David at Hebron and said to the king, “Here is the head of Ishbaal, son of Saul, your enemy, who sought your life; the LORD has avenged my lord the king this day on Saul and on his offspring.” 

The sons of Rimmon (Rechab and Baanah) took revenge on Ishbaal because he had killed one of their brothers. The fact that it served King David’s purposes was a bonus. They expected to be reward by David, but that’s not what happened. David had them killed because they had killed the man in a manner God would not approve. David didn’t want innocent blood on his hands.

What I find interesting is the contrast between how David and the two brothers Rechab and Baanah responded to enemies. The brothers took matters into their own hands. David, on the other hand, “forgave” Saul. He had two chances to kill Saul but didn’t to it. In other words, David returned to God the right to deal with Saul rather than doing so himself – something that can be really hard to do.

It’s one thing to let the offense go when there’s little opportunity for revenge. It’s another when revenge is staring you in the face.