I kill ... I wound ... I will make mine arrows drunk with blood, and my sword shall devour flesh. Deuteronomy 32:39-42
First Samuel begins where Joshua left off. God kills Eli's sons and 34,000 Israelite soldiers, smites the Philistines with hemorrhoids in their secret parts, and kills 50,070 for looking into the ark of the Lord. He thunders on the Philistines and forces them to kill each other.
But there's one story in First Samuel that everyone should know: the Amalekite massacre, in which God ordered Saul to kill every Amalekite "man and woman, infant and suckling, ox and sheep, camel and ass." Saul failed to commit complete genocide, so God gave his kingdom to David, who was always happy to kill anyone for God.
When Samuel anointed David as king, "the spirit of the Lord was with David from that day forward" (1 Samuel 16:10-13), So God was with David in each of his killings.
Samuel hacked Agag (the Amalekite king that Saul didn't kill) to death before the Lord, David bought his first wife with 200 Philistine foreskins and got another wife (Abigail) after God killed her husband (Nabal).
The book ends when God kills Saul and his three sons to punish Saul for the incomplete genocide of the Amalekites.
Here is a list of the killings in 1 Samuel.
