David comes back from slaughtering the Amalekites. He's only been home for two days when a man comes from Saul's camp with his clothes torn and dirt in his hair. Rather than running him out of town like a dirty hippie, David asks where he came from and what's happening over in Saul's camp. He fills them in on Saul and Jonathan's deaths, and also lets slip that he himself is an Amalekite and that Saul had begged him to kill him, rather than in the previous book, where he fell on his sword. He shows them Saul's crown and bracelet as proof.
David is upset and tears his own clothing. He and his men weep and fast FOR THE WHOLE DAY. Then he goes back to the stranger and confirms he's an Amalekite. He asks how he could have killed someone anointed by god without fear. Then he orders his men to kill him. He laments some more and coins the phrase how the mighty have fallen (v. 19).
He orders Saul's death be kept a secret and forbids rain or dew. He gives a nice eulogy that is about as true as any you'd hear today about how they were lovely and pleasant in their lives, and in their death they were not divided (v. 23).