The Man After God's Own Heart

The book of Acts describes David as "a man after God's own heart".

For example, the Bible records that David had killed numerous people as per 1 Samuel 27 in order to kill and to steal. On top of this, David was the one who killed Goliath, had his mistress' husband killed, and a refrain of the people of Israel was made that credited Saul with thousands of kills, but David was credited with tens of thousands (1 Samuel 18:7).

We then see that God had killed tens, hundreds, thousands, tens of thousands, hundreds of thousands, millions. One just needs to look at the Noachian Flood, the plagues of Exodus (in particular, the last one that whose specific purpose was to kill children), the numerous times God killed people who complained, people he didn't like, nations he judged, and times he pronounced familial cannibalism as a punishment.

So we see that David was indeed a man after God's own heart - God was a war god who had no problem killing people to get his own way, and David had little problem killing people when it suited him as well.

Furthermore, there are only two instances in the Bible where God has a problem with David's actions:
God's unhappiness when David slept with another man's wife and had the husband killed (2 Samuel 12), and in 1 Chronicles 28, God basically denies David a planning permit for a temple on the basis that he (somewhat ironically) "has shed much blood", however God never chastises or punishes David directly for all that killing like he did when he took away another man's wife.

It's almost like God has more of a problem with informed consensual sex than he does genocide...

So when the Bible says David is a man after God's own heart, it is entirely correct. Both have no problem killing people to get their way, both have no problem taking things for spurious reasons, and both say really nice things about each other.