Chapter 26 has no notes from the SAB, but perhaps it's a good time to pause for a moment and consider a few points that could have been brought up in earlier chapters, but I didn't yet bother with. With this chapter carrying a bit of a repititious story, it's a good time to comment.
Once again, Saul comes after David to kill him, and once again, Saul is caught in a position where he is entirely at David's mercy. Of course, once again, David spares him.
I've been arguing a lot in both my analyses and in the comments thereafter the point of self-defense. I think when a person is after you to take your life or harm your family, you have a moral right to fight back. Yes, Jesus preached that we should "turn the other cheek", but that's an idealistic command, not always a practical one. Call it a cop-out for a Christian if you will, but that's how I feel. Personally, I don't know that I could kill someone with any provocation, but I wouldn't at all look down on someone else who killed another as a defensive act.
That being said, there's a lot of violence going around in the book of 1Samuel, and many people in the story don't seem to have qualms about killing, even when not in self-defense. David arguably has basis to call the killing of Saul self-defense; surely that's what Abishai is thinking when he suggests that Saul should be killed. Not only is it a good excuse for David, but there's an added bonus that Saul's blood wouldn't be on David's hands, since Abishai is offering to do the deed himself. David tells Abishai not to touch him; why?
There are a number of possible reasons that David may be so hesitant to lay a finger on Saul. The reason David gives is that Saul is the anointed king of Israel, and as such, no Israelite (not even David himself) has the right to hurt him. Perhaps David really feels this is the case, and his given reasons are completely honest.
Other possibilities exist, however, and it may be one or more of them. Note the fact that in moments of peace between them, Saul addresses David as "son", and David in return calls him "father" (24:11). I do think that despite Saul's animosity towards this future king, David has a soft spot in his heart for Saul, both out of his love for Saul directly, and indirectly out of his love for Jonathan, and thus the vow he made to both of them that he would not enact violence on their descendants.
The most practical answer, and also the most cynical, but one not to be dismissed, is that David felt that he was setting an example in the way he interacted with Saul. After all, if some kid who thinks he ought to be the king of Israel can just sneak into the king's tent one night and bump him off, what's to stop the same from happening to David in the future? Perhaps also, almost going back full circle to the first reason, he is fearing that if he strikes down Saul, he virtually sets it up as a rule that kings of Israel will be deposed by assassination. Call it karma, justice, the will of God, or what have you, but if David does this to Saul, he's virtually bound to meet the same fate a few years down the road.
Whatever the real reason, Saul is allowed to live once again, and the two men part ways in peace, never to meet again.