Albert Barnes Commentary 2 Samuel 1:12

"and they mourned, and wept, and fasted until even, for Saul, and for Jonathan his son, and for the people of Jehovah, and for the house of Israel; because they were fallen by the sword." — 2 Samuel 1:12 (ASV)

For Saul... David’s thoroughly patriotic and unselfish character is strongly displayed here. He looked upon the death of Saul and the defeat of Israel by a pagan foe with unmixed sorrow, even though it opened the way to the throne for him and removed his mortal enemy. For Jonathan, he mourned with all the tenderness of a loving friend.