"The army took them from the Amalekites," Saul explained. "They kept the best sheep and cattle, so they could sacrifice them to the LORD your God. But we destroyed everything else."
All the animals were going to be destroyed anyway. That's why the army brought the best sheep and cattle to Gilgal as sacrifices to the LORD your God."
Saul and his army let Agag live, and they also spared the best sheep and cattle. They didn't want to destroy anything of value, so they only killed the animals that were worthless or weak.