Question: "Was the killing of Saul's descendants a just response to Saul's killing of the Gibeonites?"
Answer: Second Samuel 21:9 says that David gave seven of Saul’s descendants (two sons and five grandsons) “over to the Gibeonites, who killed them and exposed their bodies on a hill before the LORD. All seven of them fell together; they were put to death during the first days of the harvest, just as the barley harvest was beginning.” The grisly proceedings were obviously sanctioned by David. Did God also approve of the slaughter?
The background for the slaying of Saul’s descendants was this: years before, King Saul had tried to eradicate the Gibeonites from Israel; however, his action violated the covenant Joshua had made with Gibeon in Joshua 9. As a direct result of Israel’s breaking their covenant, God sent a famine upon Israel for three years. After Saul’s time, David had the responsibility to provide justice for the Gibeonites. When he asked them what they would require to make things right, the Gibeonites requested the lives of seven of Saul’s sons, and David handed them over.
It seems that Saul’s seven descendants who were killed were no better men than Saul had been: the fruit had not fallen far from the tree. Reading 2 Samuel 21:1, we see that “during the reign of David there was a famine for three successive years, and David sought the face of the LORD. And the LORD said, ‘It is because of the blood shed by Saul and his family, because he killed the Gibeonites’” (emphasis added).
Notice that the famine was not simply because of Saul’s sin but because of “his bloody house” (NASB). Seven of Saul’s descendants were killed because of their own bloodguilt. Perhaps they had aided Saul in some manner in the slaughter of the Gibeonites. As the NLT says, “The famine has come because Saul and his family are guilty of murdering the Gibeonites.”
In short, David (and God) saw the killings as justice against the “bloody house” of Saul. Yes, God did approve of the killing of the guilty in this case, because it was a just punishment for their involvement in the prior murder of innocent men.