In this chapter, Absalom takes the advice of Hushai over the advice of Ahithophel, and David escapes with his life.
In the previous chapter, Absalom accepted the advice of Ahithophel in order to fulfill the words of Nathan the prophet. In this chapter, Ahithophel and Hushai offer conflicting advice, and Absalom chooses the advice of Hushai. In effect, this validates David's strategy of leaving Hushai behind to give bad advice to Absalom, but perhaps more importantly we can see in v. 14 that it is the LORD acting, "in order to bring disaster on Absalom." So in my opinion, this chapter is not as much about David defeating Absalom through craftiness and deceit than it is about God helping David and eventually reestablishing his kingship.
Similarly, the escape of Jonathan and Ahimaaz shows more of the fortune of God than any particularly careful planning on David's part. Importantly, Jonathan and Ahimaaz are the sons of Zadok, which was the second element of David's earlier planning. David had met with four friendly groups on his way to exile. The first was his most loyal soldiers, the Kerethites and Pelethites. The second was Zadok and his sons, who are now benefiting him by bringing him news about Absalom's plans. The third was Hushai, who benefited him by leading Absalom into a poor decision. The fourth was Ziba, who brought him food and wine for his men. So basically, every group that David met has now helped him in one way or another, but overarching all of this is God's intention to protect David and bring Absalom to ruin.
Verse 14 tells us that Ahithophel's advice was good and Hushai's advice was bad. The fundamental reason why is that Ahithophel would strike quickly before David has time to escape and regroup, but in the end Absalom is afraid of David's reputation and decides to build a massive army to overwhelm David's forces.
What surprises me about this chapter is how broadly unified Israel is behind Absalom, such that he could rally an entire army from the very north to the very south of the country against its (former?) king. It's easier for me to imagine the nation changing their declared allegiance from David to Absalom, but now they are will to actively seek to kill him. So this chapter feels like a pretty drastic escalation.
One minor point I'd like to make. If my readers remember, there were several places in the Pentateuch that contained lengthy repetitions of earlier discourse. For instance, Gen 24 contains two copies of the story about the servant of Abraham meeting Rebekah. The first copy of the story is when it happens, and the second copy is when the servant is laboriously recounting that same story to Laban and Bethuel. In a similar way, the book of Exodus contains significant blocks of duplicated text regarding the tabernacle: first there is a long section describing the plans for its construction, and then there is a long section describing how the workmen constructed it exactly according to those specifications. In all of these repetitions, I noted that repetition is a common pattern in oral traditions because it aides memorization.
I mention all this because in this chapter alone there are three occurrences of a phrase translated to "such and such" or "so and so" (v. 15 and v. 21) and while it is a relatively minor thing, I think it shows us the very different origins of the Pentateuch compared to the book of Samuel. More explicitly, Samuel does not utilize repetition, which indicates that it may not have emerged as an oral tradition when it was first written. This isn't a perfect rule because we know that large portions of the OT were transmitted orally before ever being written down, but it seems likely that the Pentateuch has a longer oral history compared to Samuel. We could have figured that out just by analyzing cultural differences between Israel's history in the time of Abraham compared to David, but I think the textual difference is strong corollary evidence.
Moving on, Ahithophel kills himself when he sees that his advice was not followed. It's not clear to me why, but I always assumed it was because Ahithophel considered himself disgraced or that he was removed from influence when Absalom disregarded his advice. Possibly Ahithophel foresaw Absalom's defeat and therefore saw no purpose for living further.
What is clear, however, is that Ahithophel was under judgment from God for turning against David, and therefore his disgrace and death was a punishment for his betrayal, similar to how God is planning to bring disaster upon Absalom.
Lastly, I find it somewhat ironic that Shobi comes to assist David when David fought his brother Hanun back in 2 Samuel 10. Perhaps Shobi favors David because of the "kindness" that that Nahash showed towards David (2 Sam 10:2).
Nevertheless, this chapter ends with Absalom firmly in control over the nation, but David escapes into the wilderness with all his men and they are rallied to fight back.