Chapter 17

Ahithophel asks for 12,000 men to pursue David and those with him that very night.  He says he will kill only David, and bring the others back.  Absalom calls Hushai and asks him if he agrees.  Hushai does not, and makes a good case for waiting, gathering a very huge army, and pursuing David with them, and with Absalom himself in charge.  This way David and all those loyal to him can be killed to a man.  They all decide to take Hushai's advice.  This advice seems more sound to me also...

BUT, it says that God had ordained for the counsel of Ahithophel (which was after all the best plan) to be defeated so God could bring harm upon Absalom.

Once Hushai's advice is accepted, he sends word to David warning him to go ahead and cross the Jordan immediately, before dawn, so that David and his men cannot be penned against the river by the coming army.

When Ahithophel finds out his advice has been over ruled, he gets on a donkey, goes home, and hangs himself.  I am guessing that Ahithophel knew that following Hushai's advice would result in a victory for David, and that it wouldn't be long until David avenged himself on Ahithophel for his desertion to Absalom.  Lots and lots of intrigue.  No rest, no peace any more for David.  (Ahithophel was Bathsheba's grandfather I believe.)