I Samuel - Chapter 15

Now that the nation had settled the land and appointed a king, G-d sent word through Shmuel that it was time to fulfill the obligation of eradicating the nation of Amalek, who attacked Israel after they left Egypt. G-d commanded to destroy the entire nation of Amalek, including the livestock.

Shaul sent word to the Kenites, descendants of Yisro (Jethro, the father-in-law of Moses) who lived in Amalekite territory, that they should evacuate, after which time Shaul attacked. They destroyed the Amalekite city, but they captured Agag, king of Amalek, and kept the best livestock to use for sacrifices to G-d. G-d was displeased and he sent Shmuel to Shaul.

Shaul beamed that he had fulfilled G-d's command. Shmuel then asked, "So then what are all these cattle and sheep doing here?" Shaul replied that they were intended as sacrifices. (Agag would be publicly executed.)

"G-d told you to kill all the livestock," Shmuel countered. "Do you think He wants sacrifices or do you think He wants you to listen to what He tells you?" (An analogous case would be when a teen's parent says they can't go to a concert. If the teen sneaks out to the concert, bringing the parent back a T-shirt only adds insult to injury.) Since Shaul rejected G-d's word, Shmuel said, G-d has rejected Shaul as king. (Yes, he already cost himself the kingdom in chapter 13, but there he cost himself a lasting dynasty. Here he cost himself the throne altogether.)

Shaul admitted that he had sinned, but he tried to explain his actions. Shmuel was having none of it. He turned to leave and Shaul grabbed the hem of Shmuel's garment, which tore. Shmuel told him that this symbolized how Hashem has torn the kingdom from Shaul.

Shmuel personally executed Agag. Unfortunately, in the time between his capture and execution, Agag had a conjugal visit. From that union eventually came Haman (referred to in Megillas Esther as "the Agagite"), proving once again that G-d knows what He's talking about and we're much better off listening!