The Israelites had sworn to spare them, but Saul in his zeal for Israel and Judah had tried to annihilate them

 2 Samuel 21:1-22

During the reign of David, there was a famine for three successive years; so David sought the face of the Lord. The Lord said, “It is on account of Saul and his blood-stained house; it is because he put the Gibeonites to death.”

The king summoned the Gibeonites and spoke to them. (Now the Gibeonites were not a part of Israel but were survivors of the Amorites; the Israelites had sworn to spare them, but Saul in his zeal for Israel and Judah had tried to annihilate them.) 

“What do you want me to do for you?” David asked.

They answered the king, “As for the man who destroyed us and plotted against us so that we have been decimated and have no place anywhere in Israel, let seven of his male descendants be given to us to be killed and their bodies exposed before the Lord at Gibeah of Saul—the Lord’s chosen one.”

So the king said, “I will give them to you.”

The king spared Mephibosheth son of Jonathan, the son of Saul, because of the oath before the Lord between David and Jonathan son of Saul. But the king took Armoni and Mephibosheth, the two sons of Aiah’s daughter Rizpah, whom she had borne to Saul, together with the five sons of Saul’s daughter Merab,[a] whom she had borne to Adriel son of Barzillai the Meholathite. He handed them 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.

They buried the bones of Saul and his son Jonathan in the tomb of Saul’s father Kish, at Zela in Benjamin, and did everything the king commanded. After that, God answered prayer in behalf of the land.

Then David’s men swore to him, saying, “Never again will you go out with us to battle so that the lamp of Israel will not be extinguished.”

These four were descendants of Rapha in Gath, and they fell at the hands of David and his men.

Why?

The Lord remembers everyone who swore, so the famine happened.

David did as the Gibeonites said, let seven men live to repay the Gibeonites' blood.

David was getting old, so his soldiers asked him not to go out with them to battle.

How?

Don't sware because it is needed to make payment, we just doing the faithful thing.