This chapter marks another turning point in Saul’s life, when he goes from being disobedient to the LORD to being dangerous to the people around him.
Jonathan loves David, because they both have the same spirit. We see the same attitude in both of them that “the LORD can save by many or few”. That is a direct quotation of Jonathan, while David says that "the LORD will deliver you up into my hands" when speaking to Goliath. Both of them refer to deliverance in battle coming from the LORD.
Both David and Jonathan also show the same aggressiveness towards their enemies. David “ran quickly towards the battle line” (1 Sam 17:48) and Jonathan “climbed up, using his hands and feet” to attack the Philistines (1 Sam 14:13). We are going to see this friendship emerge as a major story element later in this book, but for now I’m just pointing out the similarities between David and Jonathan as a probable basis for their mutual friendship. Some commentators suggest that David and Jonathan get into a homosexual relationship, which I believe is unsupported by the text (not the least of which, because David later marries a woman, but more generally, because the text never says anything that infers sexual activity between them). More than anything else, I think we can see strong similarities in their personalities and attitudes, and that is almost definitely the basis of their friendship. Jonathan is also honoring David for the great victory that he just won on behalf of the nation.
Saul, however, feared David’s growing reputation. We know from many places how much Saul cared about what the people thought of him, so it is unlikely he would have been pleased by any man who was better regarded than himself. Saul is the kind of leader who does not permit anyone to be greater than himself, because that would grate against his insecurities. He finds his greatness by being superior to others, rather than by his identity in the LORD as part of the LORD’s people. This is ultimately destructive to both Saul and the people around him. Saul could have benefitted tremendously from David’s service, because David was entirely loyal to him and very successful. David, in turn, could have benefitted from having a lord who wasn’t trying to murder him, because nobody likes getting murdered.
I think it’s likely that Saul is also afraid that David will be the usurper that Samuel threatened. In v. 8 Saul appears to have (correctly) guessed that David would be the one to take his kingdom. What Saul got wrong is that he thought David would try to take the kingdom from him, perhaps in some sort of coup. Instead, by attempting to kill David and gradually driving him away, Saul is in fact undermining his own rule. What we will see in the future is that David has absolutely no intention to overthrow Saul, even after everything that Saul does to him.
Something I’ve often wondered about is why David remained in Saul’s service after Saul first tried to kill him, in v. 10-11. I’m really not sure. Perhaps David attributed Saul’s increasingly erratic behavior to the “evil spirit” and did not think that Saul harbored any genuine malice towards him. Maybe Saul apologized and insisted he would never repeat the attempt. It does appear that most of the time, Saul did not try to directly kill David. But as we can see, Saul was trying to deliberately put David into dangerous roles so that the Philistines could kill him, so in whatever respect David thought he was safe from Saul, he judged incorrectly.
Nevertheless, all of Saul’s attempts to undermine David failed. After making David a commander in Israel’s army, “all Israel and Judah loved David” as a result (v. 16). After offering Michal his daughter for 100 Philistine foreskins, David brings back 200, earning for himself the king’s daughter where Saul had plotted his death. The very things that Saul has done to kill David prepare him for eventually claiming the throne. This is the second time in as many chapters that David has brought human body parts to Saul, and if anything this time is much more disgusting than the last. Imagine how Saul must have reacted when David came to him with a sack full of human foreskins. That must have been an interesting conversation.
Some other things to note. I've mentioned a couple times how Israelite poetry works, that it uses couplets and parallelism. There are hundreds of examples of what I call thematic parallelism, and the songs of the women in verse 7 is another example of this. I don't believe the women are trying to say David is greater than Saul, what they are saying (if I may paraphrase) is "Saul has done great things and David has done great things too".
Honestly, Saul is acting like David's brothers, trying to shut down David because David is doing greater things than them. Unlike David's brothers, Saul can simply try to murder him and there is nobody who can hold Saul to account.
Another minor note is that Saul probably asked for foreskins because of the symbolic value. The Israelites are circumcised as part of their covenant with the LORD, so cutting off their foreskins as evidence of their murder (much like scalping) is probably intended to be insulting, because being uncircumcised is sort of a derogatory term in the bible. For instances, in Judges 14:3 Samson's mother asks him, "Must you go to the uncircumcised Philistines to get a wife?" Later in Judges 15:18 Samson says, "Must I now die of thirst and fall into the hands of the uncircumcised?" In all of these cases (and more that I have not referenced) the term uncircumcised is meant to be insulting, because it highlights how the Philistines are not in a covenant with the LORD, idolatrous, sinful, etc.