Now that Saul is long gone and people are trying to align themselves with David, the path seems clear for him to become king. That is exactly what happens. The tribes of Israel gather together and anoint David their king. Everyone is behind him except for those in Jerusalem where he wants to make his house. So, he sweeps in and wipes out everyone who opposes him in the city. Other kings recognize his greatness and send him supplies to make his house.
One line stands out to me in all of this though: II. Samuel 5: 13: "And David perceived that the Lord had established him king over Israel, and that he had exalted his kingdom for his people Israel's sake." What seems odd here? David PERCEIVED....Note what isn't said. It does not say that God made David king. In fact it was the tribes of Israel that made David king. God may have been with him, but it was not God that made him king. In fact, if you go back to I. Samuel, God did not want a king for the people. He only gave them a king because they kept whining and asking for one. So, we are to believe that even David is getting a little full of himself thinking that God has made him king.
David then has lots of concubines and children to establish himself and follows the advice of God in smiting the Philistines who are testing the abilities of the new King.
In Chapter 6, David has plans to bring the Ark of the Covenant to his city. Along the way though, one of his servants puts his hands on the Ark and is destroyed by God. This knocks the wind out of David's sails a little bit and makes him worried about bringing it into his city. So he puts it in someone else's house for 3 months to see what happens, as if he is giving soup to a taste-tester to see if it is poisoned. After those who are keeping the Ark are blessed by God, David brings the Ark into the city and starts dancing and partying in front of it. One of David's first wives, Michal (daughter of Saul), points out that David is making a royal arse of himself in front of his servants and their handmaids. He goes off on her, basically telling her "I was doing it for the Lord and I'll get a lot worse." He even goes so far as to tell her that she is just upset because God likes him more than he liked her dad. This seems to be a far cry from the David that wouldn't harm Saul even when given the chance because he was anointed by God.
There is a saying, "Power corrupts, absolute power corrupts absolutely." I. Samuel saw the fall of Saul because of this. We will have to keep reading to see what happens to David. (Although chp. 5 just told us he was king for 40 yrs, so this probably works itself out.)