The text gives us more nuanced data.
To be sure, Scripture characterizes David as "a man after God's own heart" (I Samuel 13:14), as one who "walked before God in integrity of heart and uprightness" (I Kings 9:4), as "God's servant who kept God's commands and followed God with all his heart, doing only what was right in God's eyes" (I Kings 14:8), and as a man "whose heart was fully devoted to the Lord" (I Kings 15:3).
All of which would incline us to accept the hagiographic version of the David narrative, except for less obvious but still intentional hints in the text that David entered into service for a Philistine king, attacking Israel as a mercenary (I Samuel 27:8, 30:26). In addition, David arranges for cold-blooded executions, ordering the slaughter of the blind and the lame (II Samuel 5:6-10), hardly battlefield heroics. He further commits adultery and arranges a murder (II Samuel 11:1 to 11:27). He hires heathen Philistines as his bodyguards (II Samuel 15:13 to 15:18), causing the reader to wonder why David couldn't trust his own countrymen to guard him. Despite earlier promises to protect the offspring of Saul (I Samuel 20:15 and 24:21; II Samuel 19:23), he orders the execution of Saul's children and grandchildren (II Samuel 21; I Kings 2:8); the one surviving grandson of Saul is placed under house arrest, so that David can keep an eye on him, lest he develop dynastic ambitions (II Samuel 9). As David lies on his deathbed, he orders a final round of executions to get rid of his political enemies.
Some efforts have been made over the centuries to bowdlerize David's biography. Some scholars argue that he did not attack Israelite cities, but merely pretended to attack them, while working as a mercenary for the Philistines; one is hard-pressed to imagine that the Philistines were stupid enough to be thus fooled, or easy-going enough to allow such behavior on the part of a hired gun. Some would argue that David was showing kindness and hospitality to Mephibosheth, but anyone familiar with the dynamics of hereditary monarchies in the Ancient Near East will understand that any living offspring of a former monarch was immediately and universally understood to be a threat to the life of the current monarch; Mephibosheth's mere existence was effectively an assassination attempt on David. David was acting on the old maxim that one should keep one's friends close, but one's enemies closer.
Despite this list of sins, Scripture keeps David as an example of a godly man. His son, Solomon, is judged (II Kings 11:4) as not having lived up to the Davidic standard:
when Solomon was old his wives turned away his heart after other gods, and his heart was not wholly true to the Lord his God, as was the heart of David his father.
We have here a hint as to why the text is willing to overlook David's many and significant sins. David did not commit idolatry. David's life, including his sins, and especially his sins, was lived in the consciousness of the one true God. It is to this Lord that David repents.
One might think that David retains his status as role-model in the Bible because of his political and military exploits; he has been, at times, a sort of folk hero to the Israelites. Although some of the Jews may have in fact retained David as a hero for that reason, it is for different reasons that the text retains him as a spiritual template.
It is worth noting that David called Jerusalem "the City of David" (II Samuel 5:9), whereas God called Bethlehem "the City of David" (Luke 2:4 and 2:11). God focuses not on David's hero status, but on David's spiritual status. The Messiah would be called the "Son of David" - the Israelites took this to indicate David's worldly exploits; God took it to mean a man of faith. The Jews connected David to Jerusalem - he made it into a political and ecclesiastical capital - but Jesus found Jerusalem distasteful (Ezekiel 16:48, Matthew 23:37, Luke 13:34).
David is not the hero of the narrative. God is the hero. David is a flawed, finite, and sinful man. God uses David, and through David, God accomplishes great things. David is an instrument; by means of David, God works. There is good news in this. If God was willing to use a man so sinful and flawed as David was, then God will be willing to use people so sinful and flawed as I am, and as you are.
