Shalom,

The Ari *R.Isaac Luria* explained that actually the "sin" of David wasn't the taking of Bat-Sheva since she was his intended, but by not executing her husband for acting against the authority of King David.  All warriors at the time had open letters of divorce to their spouse so that the spouse was free to marry another man should the husband not return from the war.

King David allowed the "ex" husband of Bat Sheva to go battle on the frontlines where his death was a form of idol worship as the blood of the enemy was used as a form of prize and ritualistic magic was employed with it through the weapons of the enemy army.  The proper form of action according to the Ari would've been to simply have Bat-sheva's husband executed for rebellion.
Of course to our modern sensibilities this seems a bit convenient to excuse the killing of a man whose wife a king would want.  But the Zohar and several other sources clearly point out that Bat -Sheva was David's intended match from before time.

Best Wishes----

R.Yosef Karpman