2 Samuel 1-3: How the Mighty Have Fallen

You would have thought that David would have been rejoicing and celebrating when he was told that Saul had been killed, but instead he tore his clothes and mourned and wept until evening.  David was mourning not just because he had lost his closest friend, Jonathan, but he mourned for Saul as well.  "Your glory, O Israel, lies slain on your heights. How the mighty have fallen!  2 Sam. 1:19 NIV  He took the death of Saul and Jonathan so hard, that he killed the messenger who told him of Saul's death. 

And when he was anointed King over Judah, it was his men more than David himself who pursued the rest of Israel and the house of Saul until it was wiped out and David was proclaimed king over all of Israel.  The men surrounding David were not as trusting as he was.  First they murdered Abner, the commander of Saul's army and caused David to grieve.  Once more he put on sackcloth and ashes and walked behind Abner's bier.  His grieving proved to be a consolation to the people of Israel, but it was not a political calculation on David's part, he truly grieved for Abner.  Then the king said to his men, "Do you not realize that a prince and a great man has fallen in Israel this day? - 2 Sam. 3:38 NIV

Then, after the death of Abner, some others took it upon themselves to murder Saul's remaining son, Ish-Bosheth.  When they brought the news of their revenge to David he rewarded them with the same reward he had given the messenger who brought the news of Saul's death.  David, the young, but wise, thirty year old king, knew what his men did not.  He knew the heart of God.  It was as if he had been sitting with the crowd on the mountain as they listened to Jesus' words.
"Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;"  - Matt.5:43-44 NIV