Saul spared the king and he saved some of the cattle to be used for his own purposes. In consequence, and for other reasons, the priests, inspired by God, selected David and backed him in a civil war that ended with the overthrow of Saul. In all the discussions of Saul's transgressions, I remember no voices raised to question the morality of demanding wholesale slaughter of the population of a conquered land. These people who accepted the mass murder of the Amalekites in stride were good Americans. They would properly shrink in horror at the thought of a modern campaign to exterminate the entire civilian population with which their country might go to war.
All religions evolve.
All nationally recognized religions began as primitive expressions, their gods were first tribal gods. As their spheres of influence expand, worshipers' perceptions of their gods of necessity evolve to become more inclusive. As with any change, as the perception of a god changes, there are worshipers who see any evolvement as corruption of the faith. They demand a return to the true god and his pre-evolution admonitions. Americans are asking why religious fanatics would murder over 6000 innocent civilians in an act of terrorism.