God has his revenge on for the Amalekites and orders Saul to kill them all, including the old and infirm and their animals. Remember, he thinks he's a merciful and just deity. Saul gathers an army of 200 000 which is 24 000 more troops than are enlisted in the current Israeli army and goes and hides in the valley. He warns the Kenites to leave, lest there be confusion in the battle.
Saul mostly does as he's told, but spares the king and a few of the best animals. God finds out of course, and bitches to Samuel about his choice of king. Samuel cries. When he finishes, he takes Saul out for a walk and asks about the animals. Saul, realizing the jig is up, claims they're for sacrificing. Samuel rebukes him, saying that god wants obedience more than meat, but Saul is stubborn and refuses to concede. Samuel eventually demotes him. Saul doesn't seem very upset by the news, he just asks Samuel to pray with him.
After that, Samuel summons Agag, the Amalekite king. Agag asks for his life, but Samuel hacks him to death in front of god. This is exactly the kind of deity I want on my side!
The two men go home then, and Samuel never calls on Saul again.