I would invite you to read the first online chapter of Gary Greenberg's new book on King David. You can google his name for his website. Now he killed many of Saul's line I dont know if every single one was killed. Saying God picked David to be king is like saying god picked Obama over McCain.
Greenberg asserts:
READ CHAPTER ONE
Good David and Bad David
But the word of the Lord came to me [i.e., David], saying,
Thou hast shed blood abundantly, and hast made great wars:
thou shalt not build an house unto my name, because thou hast shed
much blood upon the earth in my sight. (1 Chronicles 22:8.)
Modern impressions of King David depict him as a young boy of unsurpassed courage, a pious and humble man who triumphed over many adversities, a goodly king whose heart was with the Lord and compared to whom all other monarchs fall short. He is portrayed as the progenitor of a dynasty that would one day rule over the kingdom of God on earth. Many see in him history’s first renaissance man: a poet of uncommon wit and intelligence, a musician of national renown, a diplomat of consummate skill, a politician of great wisdom, a brilliant military strategist, a master of martial weapons, a theologian who defined the basics of Jewish thought, and an inventor of many of the Old Testament’s religious practices and institutions. These views have a long pedigree.
Following strict biblical chronology, King David came to the throne at about 1061 b.c., but the biblical data presents many problems, including textual contradictions and problematic synchronization with the dates of known events from non-biblical records. Most scholars propose moving the start of his reign forward about fifty or sixty years, somewhere between about 1010 and 1000 b.c.
David’s predecessor on the throne was Saul, the first king of Israel, and David’s successor was Solomon, his son. By convention, biblical scholars refer to the period from Saul through Solomon as the United Monarchy. It is usually thought of as ancient Israel’s golden age and the three kings, according to the biblical chronology, had a combined reign of about one century.
David and Saul came from different families and rival political entities that shared territorial borders, Saul from the tribe of Benjamin and David from the tribe of Judah. When David succeeded to the throne, he founded a family dynasty that lasted over four hundred years, perhaps longer than any other known royal family. Although David’s kingdom split in two after the death of Solomon, Israel in the north and Judah in the south (mirroring the earlier political divisions between Saul and David), his descendants on the throne of Judah outlasted the more popular and prosperous northern kingdom by almost one hundred and fifty years.
His dynasty ended in 587 b.c. when Babylon captured the Judaean capitol of Jerusalem and destroyed the great Temple of Solomon, but Hebrew prophets believed and predicted that a future descendant of David would once again rule over the kingdom of God. Christians saw the fulfillment of that promise in Jesus Christ while Jews continue to await the arrival of the Messiah.
In the second century a.d., after the Romans banished the Jewish people from Palestine, the Hebrew academy in Babylon emerged as the intellectual and spiritual center of Jewish culture throughout the Diaspora. For almost a thousand years a leader believed to be a descendant of David presided over that institution. As Christianity took hold in Europe, the ideology and theology surrounding King David inspired many Christian monarchs and religious leaders and triggered many political and religious struggles between Christian kings and the Popes.
These views of King David, burnished over millennia by armies of theologians and religious teachers, have made David the most beloved character in all of the Old Testament and, theologically, the most important. But how much do we really know about this man?