Our Scripture reading today is 2 Samuel 1:17-25.
Verse 19, "The beauty of Israel is slain upon thy high places: how are the mighty fallen!"
The nation of Israel had suffered a great loss.  Both King Saul and his son, Jonathan, had been killed in battle.  Inspite of how King Saul had tried to kill David on multiple times, David speaks highly of him and instructs there to be a time of mourning.  As we read this verses, we can see into David's heart of compassion for Saul and Jonathan.  
I think it would be safe to say that King Saul at one time was an enemy of David.  God's Word gives us specific instructions on how we are to treat our enemies.  In fact, it was Jesus Christ preaching in Matthew 5:44, "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;"  I must say that in order to fulfill this verse, we need to be walking in the Spirit.  It is not natural to do what Jesus is preaching.  The natural reaction is to return exactly how we are being treated.  
How could David speak this way about someone who tried to kill him?  David was walking with the Lord and had already been anointed the next king.  He needed to set the right example for those who would be watching him.  There will be many times in our life when we have the same opportunity to show others how to treat people with the compassion they may or may not deserve.