The Amalekite Genocide

In the old testament God says (1 Samuel 15) "... Now go attack the Amalekites and totally destroy all that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys."

It is absolutely incomprehensible for me how anyone want to even claim that books containing passages (plural), such as the one above, can be a source of moral authority. Even we assume that this is not what literally happened, I still think that it disqualifies the bible as a source of morality... Also listen to the unbelievable discussion about the Amalekite genocide between christian John Allister, and atheist Justin Schieber from Reasonable doubts.