Samuel hacks Agag to pieces before the Lord

Saul ruined God's last killing by not killing all the Amalekites. God wanted a complete genocide, and Saul blew it by keeping one man alive (along with a few animals that he planned to kill later for God). Poor Samuel was so upset that he tore his skirt and cried all night. 

It grieved Samuel; and he cried unto the LORD all night. 1 Samuel 15.11

Samuel … laid hold upon the skirt of his mantle, and it rent. 15.27

But according to the story, it wasn't God that told Saul to commit genocide. It was Samuel. Samuel said "thus saith the Lord" and Saul was stupid enough to believe him. And believers have followed along ever since.

After the genocide, Samuel just kept making nasty shit up and blaming it on God.

Then came the word of the LORD unto Samuel, saying, It repenteth me that I have set up Saul to be king: for he is turned back from following me, and hath not performed my commandments. 15.10-11

The LORD sent thee on a journey, and said, Go and utterly destroy the sinners the Amalekites, and fight against them until they be consumed. 15.18

The commandment of God that Saul broke was "Thou shalt kill everyone, man and woman, infant and suckling, ox and sheep, camel and ass."

Thus saith the Lord of hosts … Go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass. 15.2-3

So Samuel had to finish the holy genocide by hacking Agag to pieces before the Lord.

Then said Samuel, Bring ye hither to me Agag the king of the Amalekites. And Agag came unto him delicately. And Agag said, Surely the bitterness of death is past. And Samuel said, As the sword hath made women childless, so shall thy mother be childless among women. And Samuel hewed Agag in pieces before the LORD in Gilgal. 15.32-33

Samuel was a monster. But he was a hero to God and still is to all Bible believers.

 God's next killing: In the valley of Ellah -- Goliath