Don't judge a book by its cover is a phrase that gets thrown around alot, but many times that is just what we do. We focus on the outward appearance and neglect what is going on in the heart. Just becuase someone has the appearance of a right relationship with God doesn't mean that they have it all together spiritually.
There were many people there that day, but David was the only one that got into the battle. Everyone else allowed their fear to keep them on the sideline, but not David. There is a spiritual battle raging all around us. Are you active in the battle, or are sitting on the sideline like everyone else was that day?
David let his guard down on his heart, and he fell into sin. And we had better understand that if the man after God's own heart is capable of adultery and murder, then we are capable of all kinds of wickedness. We need to set up safeguards in our lives so that we will not fall into sin.
David had repented of his sin with Bathsheba and had found forgiveness, but there were still repercussions for what he had done. Every choice that we make is vitally important. If we make the wrong choice, then there are issues that we will have to deal with for the rest of our lives.
Mephibosheth was a rival to the throne of David, but instead of killing him David extends him mercy. Not because he deserved it, but because of the relationship that David had with Jonathan. He was receiving good, not on his own merit, but on Jonathan's merit. We are like Mephibosheth in a lot of ways.