1 Samuel - Chapter 15

This chapter contains one of the hard passages in the bible -

"This is what the Lord Almighty says: “I will punish the Amalekites for what they did to Israel when they waylaid them as they came up from Egypt. 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.”’

God has a plan to save the world through his chosen people, and everything in the Bible up to now has been demonstrating that this is going to be fulfilled in Jesus.  The Amalekites have, for hundreds of years, been determined to destroy God's people and stop his plan.  

God is removing the identity of the people opposing his plan, to ensure the salvation of the world through Jesus.