2 Samuel 22:3, David Avenges His Parents’ Death
David also defeated the Moabites. He made them lie down on the ground and measured them off with a length of cord. Every two lengths of them were put to death, and the third length was allowed to live.
//So David conquers the Moabites, and the king of Moab dies here. Oddly, the last thing we heard about this anonymous king before David’s vengeful slaughter, was that David was asking this king to provide protective custody for his parents! What happened to tear their alliance apart?
All we have is a sort of clue from silence. We hear no more about David’s parents. So, Rabbinic and medieval Jewish commentary blame this king for the their deaths!
I guess it makes as much sense as any other explanation for David’s change of heart.