In this chapter, David spares Saul's life.
This is such an incredible chapter. David, driven away from his wife and home, running circles around a mountain trying to avoid Saul's armies, having literally dodged Saul's spear on several occasions, is now remarkably put into a position that he could kill Saul and he does not do it.
First of all, even though this chapter doesn't mention God doing anything, I think it's clear that Saul going to "relieve" himself in the same cave that David was hiding is more than a coincidence. If I may speak plainly, this opportunity is a test of David's character just as much as the trials that he's faced surviving in the wilderness, which is just as much of a trial as the times of favor and success when he was leading Israel's armies. All of these situations, both good and bad, test David by revealing his character and what decisions he will make through both honor and adversity.
Let's remember the circumstances of this meeting. Not only has David been anointed by Samuel to be the next king, not only has Saul tried to murder David several times, but Saul has actually murdered several hundred people from the priestly families in Nob. And David knows it because of Abiathar who escaped from the massacre. In spite of all this, and knowing that he would be the next king, David insists that he will not raise his hand against Saul. Instead, David is waiting for the LORD to install him as king in the proper time and proper way. I think it's worthwhile to draw a contrast between David's action here and Abraham's action when he had a child through Hagar in an attempt to fulfill God's promise that he would have a son. Even David's men think that this is an opportunity from the LORD for David to fulfill Samuel's promise to him.
All these things taken together, David's action are very honorable, though it's not clear to me that killing Saul would have been a sin considering the evil that Saul is inflicting on the nation and David in particular. But I think David's response shows such restraint and valor that it deserves our respect. I don't think it's wise to make generalizations about this chapter, although certainly some people try to do so. For instance, when we contrast David with Dietrich Bonhoeffer (a Lutheran pastor who was involved with a plot to assassinate Adolf Hitler during World War II), can we say that Bonhoeffer did wrong? Or can we say that David did wrong? Who is to judge men when put under such pressure, both for their own lives but also confronted by such a dark evil in the man who is supposed to be their leader? I don't think it's my place to judge either David or Bonhoeffer, nor do I think any living person can judge them without facing a similar situation. I have respect for David, who chose non-violence, and I have respect for Bonhoeffer, who chose to help the conspiracy against Hitler's life. I think this is a really difficult issue and I think the only true answer will come by asking for guidance from the Holy Spirit to lead us into truth. I hope I never have to decide whether to harm one person and saving another, but if I do, the only way I feel like I could make the right choice is if I am guided by the Holy Spirit.
On top of all that, I find David's language to be extremely respectful and deferential. He refers to Saul as "my lord the king", "my father", and himself as a "dead dog, a single flea" (dogs are unclean animals under the laws of Judaism, and frequently used a derogatory term). David appeals to Saul, showing the hem of his robe as proof that he could have killed Saul but didn't. Ultimately, David appeals to the LORD to judge between them and avenge the mistreatment David has received from Saul.
For the moment, Saul appears to realize the horror of what he's done to David and cries and apologizes. Perhaps more remarkably, he also predicts that David will be king. Jonathan was doing this just a chapter ago, and David's men also seem to think that he will become king. Everyone seems to know that David will become king, and now it's just a matter of time.
What always surprised me the most about this chapter though, is that after David and Saul appear to be reconciled, David goes back to the stronghold instead of returning with Saul. I always thought that Saul crying and repenting for treating David poorly meant that David could go back to Bethlehem and his wife. Instead, we see David go back into hiding as if nothing had happened. Even more remarkably, two chapters from now Saul will begin pursuing David anew, as if David had never spared his life. All I can think to say about this is that Saul is allowing his paranoia and fear of losing power to guide him into madness, and David refuses to go back with Saul because he realizes how unstable and threatening Saul has become.