Finally in chapter 5, David becomes king over all of Israel, and remains king for 40 years. This prompts the SAB to see a possible contradiction in certain events that occur in the reign of David, particularly the eventual rebellion of Absalom. I don't think I buy the logic here; if anything, it almost seems worse than the SAB is insinuating. 2Sam. 15:7 says "And it came to pass after forty years..." I say, counting from when? In the immediate context, it sounds like Absalom spent 40 years kissing up to the people, which could have happened starting before David became king, or a long time afterwards, as one might wonder how he could be doing the stuff described in that chapter while his father was on the run from Saul. On the other hand, the 40 years in the context in this chapter could mean that David was king for 40 years until he was deposed temporarily by Absalom. In any case, I don't see any of this dating to be clear enough to base a contradiction on.
Apparently, one of the first things that David has to deal with as king is some sort of challenge on his authority by the Jebusites. On the face of it, it does sound fairly cruel, but I'm not sure what's going on here at all. Are the Jebusites telling David he has to kill his own people? Are they using their own disabled population as human shields? I don't know, so I'm not sure how to comment on it. The SAB calls it violent and unjust, and I'm inclined to agree that far.
David builds himself some sort of palace, and takes more women, neither action being commented upon here, but I believe the latter being condemned elsewhere. With more women come more children, and a list is given here; the SAB points out this list doesn't match other lists, and once again, what can I say but they sure don't match. It may be that none of the lists is meant to be exhaustive, but then one might ask why, and why the particular names on each list? I certainly don't know.
So the Philistines come up and move in on David, and David inquires of the Lord if he should respond. God says yes, and vows to help, exactly as the SAB says. As I've said before, if the land is being invaded, why shouldn't they defend themselves and why shouldn't God help them do so? And when they find the Philistines' idols left behind, why shouldn't they burn them?
The Philistines come back, and this time God gives David some tactical advice, which for some reason the SAB finds absurd, but who would be better at giving tactical advice than an omniscient being?