During David’s reign, there was a famine in Israel. After it had gone on for three years, David asked God why there was a famine. God explained that he was punishing dead king Saul for trying to kill all the Gibeonites after Joshua had promised they wouldn’t be killed.
King David asked the remaining Gibeonites how he could make amends. They said they would like it if he helped them kill seven descendants of Saul. (Whose whole family had already been killed off.)
So David, who had sworn not to kill off the descendants of Saul, handed over two sons and five grandsons of Saul to the Gibeonites to be killed.1 Now that Saul’s innocent descendants had been murdered, God (who didn’t want David to kill Saul’s relatives) was willing to answer his people’s prayers and end the famine.
The end.
The moral of the story
If you promise not to kill people and then kill them anyway, God won’t punish you. He will reward you. But he might punish your descendants later for what you did.