But the king spared Mephibosheth, the son of Jonathan the son of Saul, because of the LORD's oath that was between them, between David and Jonathan the son of Saul.

Wesley's Notes

21:7 Spared - For the Gibeonites desiring only such a number, it was at David's choice whom to spare. Of Jonathan - This is added, to distinguish him from the other Mephibosheth, ver.8.

Matthew Henry's Concise Commentary

21:1-9 Every affliction arises from sin, and should lead us to repent and humble ourselves before God; but some troubles especially show that they are sent to bring sin to remembrance. God's judgments often look a great way back, which requires us to do so, when we are under his rebukes. It is not for us to object against the people's smarting for the sin of their king; perhaps they helped him. Nor against this generation suffering for the sin of the last. God often visits the sins of the fathers upon the children, and he gives no account of any matters. Time does not wear out the guilt of sin; nor can we build hopes of escape upon the delay of judgments. If we cannot understand all the reasons of Providence in this matter, still we have no right to demand that God should acquaint us with those reasons. It must be right, because it is the will of God, and in the end it will be proved to be so. Money is no satisfaction for blood. It should seem, Saul's posterity trod in his steps, for it is called a bloody house. It was the spirit of the family, therefore they are justly reckoned with for his sin, as well as for their own. The Gibeonites did not require this out of malice against Saul or his family. It was not to gratify any revenge, but for the public good. They were put to death at the beginning of harvest; they were thus sacrificed to turn away the wrath of Almighty God, who had withheld the harvest-mercies for some years past, and to obtain his favour in the present harvest. In vain do we expect mercy from God, unless we do justice upon our sins. Executions must not be thought cruel, which are for the public welfare.