When I read 1 and 2 Samuel, I am always confused about why David can be the real King? Even Saul made wrong things, but David was the same. He also made wrong things in 2 Samuel and he commited adultery against Bathsheba. But little and little, through God's words, I know our Lord looks at the heart, the repent heart. Let us compare 1Samuel 15:14-21 and 2 Saumel 12:13, we can see the different response when the prophet pointed their mistakes. Saul tried his best to quibble but David admited his mistake immediately when Nathan pointed his fault.
So this is the reason why David is the King not Saul I think! God looks at the heart, he knows our heart---our repent heart.
Hope this quote from bible could remind us to check our heart always. Let God give us a clean and repent heart all the time.