Albert Barnes Commentary 2 Samuel 21:7

"But the king spared Mephibosheth, the son of Jonathan the son of Saul, because of Jehovah`s oath that was between them, between David and Jonathan the son of Saul." — 2 Samuel 21:7 (ASV)

The Lord’s oath - The calamity brought upon Israel by Saul’s breach of the oath to the Gibeonites would make David doubly careful in the matter of his own oath to Jonathan.