This is an episode that everyone should know about. In it, God commands Saul to kill every man, woman, child, infant, baby, and animal among the Amalekites. And Saul sends an army of 210,000 to do that.
They kill everyone - except for the king (Agag) and a some of the animals to sacrifice to God. But God was angry at Saul for committing incomplete genocide on the Amalekites. And for that reason, God will take his kingdom from him and give it to someone who would be more than happy to do such a thing.