The Lord gave David victory wherever he went

As I mentioned in the introduction, God approved all of David's killings (except for the matter of Uriah).

David did that which was right in the eyes of the LORD, and turned not aside from any thing that he commanded him all the days of his life, save only in the matter of Uriah the Hittite. 1 Kings 15:5

He even helped out with most of them. Still, some of the ones from 2 Samuel 8-10 are just too damned boring to deal with separately, so I'm lumping them together here.

2 Samuel 8 begins by telling us that David somehow found some more Philistines to smite.

It came to pass that David smote the Philistines, and subdued them. 2 Samuel 8:1

Then David smote Hadadezer, the king of Zobah, "taking" 1000 chariots, 700 (or 7000, if you believe the story in 1 Chronicles 18:3-4) horsemen, and 20,000 foot soldiers. [Since the text doesn't say that David killed the 20,700 (or 27,000) soldiers, I'n not counting them as killings.]

David smote also Hadadezer ... king of Zobah ... And David took from him a thousand chariots, and seven hundred horsemen, and twenty thousand footmen. 2 Samuel 8:3-4a

David smote Hadarezer king of Zobah ... And David took from him a thousand chariots, and seven thousand horsemen, and twenty thousand footmen. 1 Chronicles 18:3-4

David hamstrung ("houghed" in the KJV) all but 100 of the 1000 horses.

David houghed all the chariot horses, but reserved of them for an hundred chariots. 2 Samuel 8:4b

Then he killed 22,000 Syrians. "And the Lord gave David victory wherever he went." (NIV)

When the Syrians of Damascus came to succour Hadadezer king of Zobah, David slew of the Syrians two and twenty thousand men ... and the Syrians became servants to David, and brought gifts. ... And the LORD preserved David whithersoever he went. 2 Samuel 8:5-6

And another 40,700 Syrians (or was it 47,000?).

The Syrians fled before Israel; and David slew the men of seven hundred chariots of the Syrians, and forty thousand horsemen. 2 Samuel 10:18

But the Syrians fled before Israel; and David slew of the Syrians seven thousand men which fought in chariots, and forty thousand footmen. 1 Chronicles 19:8

So I'll give David (and God) credit for 66,850: 1000 Philistines and 65,850 Syrians. (22,000 in the first killing and 43,850 in the second, taking the average of 47,000 and 40,070 from the stories in 2 Samuel 10 and 1 Chronicles 19.)

The Friendly Atheist - How many people did God kill in the Bible?

The Lord gave David victory wherever he went.