In this chapter, Saul orders for Ahimelech and his entire family to be killed for helping David.
The first 3-4 verses breeze over a couple important things, so I will specifically call them out. In verse 1, David's family goes to be with him in the "cave of Adullam". Presumably this is a cave somewhere in the wilderness away from the settled areas and Saul's influence. But even more interesting, David's father and brothers are now going to him. This is quite a reversal from their previous disdain for him; now David has become the "captain over them". I can think of two possible reasons for this. First of all, since David is now a persona non grata, it is very possible that Saul would murder his family simply because they are related to him. They probably go to David because they are under almost the same threat of death that he is. Secondly, this probably also reflects a newfound recognition that David has become a formidable leader in his own right. He has been a leader of Israel's armies for some time and a close servant of the king, and I think his family has finally come to recognize his authority.
This is the only passage where we really hear anything about David's family for a long time, and it's very muted, but I think it's significant because it shows us how the people who used to be most antagonistic to David are now supporting him and looking to him for leadership.
After his family, we see an assortment of troubled figures join him, all who were distressed, discontented or in debt. There are a couple of things that are interesting about this. First of all, somehow all of these people needed to figure out where David was, without Saul finding out. I don't really know how that happened, but probably people saw David going into the wilderness and somehow the word spread. Secondly, I think it's interesting that these distressed people would even think of going to join David in the first place. Ahimelech, for instance, did not know that David was on the run from the king, but now it seems that all of these paupers and runaways have discovered that David is an enemy of the king and (as the king represents their oppression) becomes their ally.
So I think that whole evolution in David's life is at least as interesting as his departure from Gibeah and Bethlehem. Even though David is the future king, his only associates are the outcasts of society, the people who have nothing else to lose, and himself an outcast, he has become their leader, even as he led the armies of Israel in the days when he was honored amongst the nation.
In verses 3-5 David brings his family to the king of Moab, which is a nation hostile to Israel. He's trying to protect his parents from Saul's inevitable retaliation. The king of Moab, in his turn, is no friend of David, but wants to undermine Israel and he probably sees David as an insurgent that will weaken Saul by causing internal conflict. Moab likely hopes to benefit by strengthening David as a force against Saul. This is not David's intention at all, but any force that Saul employs hunting David will weaken him against the surrounding nations such as Moab.
Finally, David is staying in "the stronghold", but the prophet Gad (an entirely new figure) instructs him to return to Judah, and David does so. Since "the stronghold" is not in Judah, it is probably in Moab or maybe Philistine territory. Either way, by moving back into Judah David is putting himself back into harm's way, because he is now within Saul's reach.
Saul, for his part, is sitting in Gibeah (a city in Benjamin), surrounded by his men whom he addresses as Benjamites. Saul is himself a Benjamite, and I think what we can tell from this is the power of tribal affiliations in Israel. It's a topic I've belabored previously, and with good reason because it's exactly what's going on here. Saul has promoted his family and tribe members to positions of power in the army and gifted them wealth and land. It's shameless nepotism, but that's how a lot of things worked in the ancient world (not to mention, the modern world).
Doeg the Edomite is the odd man out amongst these Benjamites, because Edom is also hostile to Israel. It is a strange thing that an Edomite should be employed in Saul's inner circle. He is the chief shepherd, which as I have previously discussed is an occupation of ill repute at this time in Israel's history. Saul might have hired an Edomite because there weren't any Israelites who wanted to do the job.
Anyway, Doeg tells Saul what he saw in the previous chapter, and after confronting Ahimelech, Saul orders his men to kill all the priests. His men refuse to do so, but Doeg (an Edomite who does not worship the LORD) is willing to kill all of these innocent men at the king's order, including all of their families, animals, wives and children.
This also shows the growing paranoia of Saul, who is not just trying to kill David, but now anybody who he suspects of helping David. It's disturbing that Saul would give his men an order so evil that the only person willing to carry it out is an uncircumcised foreigner. I think this chapter marks the point at which Saul has completed his descent into evil. There is only a single survivor from the priestly family, and like all the other desperate and distressed men, he runs to David for help and leadership. David accepts him because David realizes that he is responsible for Ahimelech's death.