The Amalekite (incomplete) genocide
1 Samuel also said unto Saul, The LORD sent me to anoint thee to be king over his people, over Israel: now therefore hearken thou unto the voice of the words of the LORD.
2Thus saith the LORD of hosts, I remember that which Amalek did to Israel, how he laid wait for him in the way, when he came up from Egypt.
3Now 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.
Samuel said to Saul,
God made you king. Obey him.
This is what he says:
I remember what Amalek did to Israel. 
Now go smite the Amalekites.
Destroy everything that they have.
Kill them all: men, women, infants, sucklings, ox, sheep, camel, and ass.
4 And Saul gathered the people together, and numbered them in Telaim, two hundred thousand footmen, and ten thousand men of Judah.
5 And Saul came to a city of Amalek, and laid wait in the valley.
6 And Saul said unto the Kenites, Go, depart, get you down from among the Amalekites, lest I destroy you with them: for ye shewed kindness to all the children of Israel, when they came up out of Egypt. So the Kenites departed from among the Amalekites.
So Saul sent 210,000 soldiers to kill the Amalekites.
7And Saul smote the Amalekites from Havilah until thou comest to Shur, that is over against Egypt.
8 And he took Agag the king of the Amalekites alive, and utterly destroyed all the people with the edge of the sword.
9 But Saul and the people spared Agag, and the best of the sheep, and of the oxen, and of the fatlings, and the lambs, and all that was good, and would not utterly destroy them: but every thing that was vile and refuse, that they destroyed utterly.
Saul killed all the Amalekites, except for the king (Agag) and some of the more valuable animals. 
God rejects Saul as king (for not killing all the Amalekites)
10Then came the word of the LORD unto Samuel, saying,
11It 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. And it grieved Samuel; and he cried unto the LORD all night.
12 And when Samuel rose early to meet Saul in the morning, it was told Samuel, saying, Saul came to Carmel, and, behold, he set him up a place, and is gone about, and passed on, and gone down to Gilgal.
God said to Samuel,
I regret making Saul king, since he disobeyed my commandment. 
And Samuel cried to God all night.
13 And Samuel came to Saul: and Saul said unto him, Blessed be thou of the LORD: I have performed the commandment of the LORD.
The next day when Samuel and Saul met, Saul said,
I did what God commanded me to do.
14And Samuel said, What meaneth then this bleating of the sheep in mine ears, and the lowing of the oxen which I hear?
Oh really? Why do I hear sheep bleating and oxen lowing?
15 And Saul said, They have brought them from the Amalekites: for the people spared the best of the sheep and of the oxen, to sacrifice unto the LORD thy God; and the rest we have utterly destroyed.
We kept the best sheep and oxen to sacrifice to God.
We killed everything else.
16 Then Samuel said unto Saul, Stay, and I will tell thee what the LORD hath said to me this night. And he said unto him, Say on.
17 And Samuel said, When thou wast little in thine own sight, wast thou not made the head of the tribes of Israel, and the LORD anointed thee king over Israel?
18 And 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.
19Wherefore then didst thou not obey the voice of the LORD, but didst fly upon the spoil, and didst evil in the sight of the LORD?
20 And Saul said unto Samuel, Yea, I have obeyed the voice of the LORD, and have gone the way which the LORD sent me, and have brought Agag the king of Amalek, and have utterly destroyed the Amalekites.
21 But the people took of the spoil, sheep and oxen, the chief of the things which should have been utterly destroyed, to sacrifice unto the LORD thy God in Gilgal.
22 And Samuel said, Hath the LORD as great delight in burnt offerings and sacrifices, as in obeying the voice of the LORD? Behold, to obey is better than sacrifice, and to hearken than the fat of rams.
God commanded you too utterly destroy the Amalekite sinners.
Why didn't you obey him?
23 For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the LORD, he hath also rejected thee from being king.
24 And Saul said unto Samuel, I have sinned: for I have transgressed the commandment of the LORD, and thy words: because I feared the people, and obeyed their voice.
25 Now therefore, I pray thee, pardon my sin, and turn again with me, that I may worship the LORD.
26 And Samuel said unto Saul, I will not return with thee: for thou hast rejected the word of the LORD, and the LORD hath rejected thee from being king over Israel.
Because you rejected the word of God, God rejects you as king. 
27 And as Samuel turned about to go away, he laid hold upon the skirt of his mantle, and it rent.
28 And Samuel said unto him, The LORD hath rent the kingdom of Israel from thee this day, and hath given it to a neighbour of thine, that is better than thou.
Samuel said to Saul,
God has torn his kingdom from you today and has given it to a neighbor that's better than you.
29 And also the Strength of Israel will not lie nor repent: for he is not a man, that he should repent.
God is not a man; he doesn't lie or repent. 
30 Then he said, I have sinned: yet honour me now, I pray thee, before the elders of my people, and before Israel, and turn again with me, that I may worship the LORD thy God.
31 So Samuel turned again after Saul; and Saul worshipped the LORD.
Samuel hacks Agag to death before the Lord
32Then 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.
Bring Agag to me.
Agag said to Samuel,
Surely the bitterness of death has passed.
33And 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.
34 Then Samuel went to Ramah; and Saul went up to his house to Gibeah of Saul.
As the sword has made women childless, so your mother will be childless.
Then Samuel hacked Agag in pieces in front of God. 
35 And Samuel came no more to see Saul until the day of his death: nevertheless Samuel mourned for Saul: and the LORD repented that he had made Saul king over Israel.
Samuel never saw Saul again. 
And God repented that he had made Saul king.