Samuel said to Saul, "I am the one the LORD sent to anoint you king over his people Israel; so listen now to the message from the LORD. This is what the LORD Almighty says: 'I will punish the Amalekites for what they did to Israel when they waylaid them as they came up from Egypt. Now go, attack the Amalekites and totally destroy everything that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys.'"
So Saul summoned the men and mustered them at Telaim—two hundred thousand foot soldiers and ten thousand men from Judah. Saul went to the city of Amalek and set an ambush in the ravine. Then he said to the Kenites, "Go away, leave the Amalekites so that I do not destroy you along with them; for you showed kindness to all the Israelites when they came up out of Egypt." So the Kenites moved away from the Amalekites.
Then Saul attacked the Amalekites all the way from Havilah to Shur, to the east of Egypt. He took Agag king of the Amalekites alive, and all his people he totally destroyed with the sword. But Saul and the army spared Agag and the best of the sheep and cattle, the fat calves and lambs—everything that was good. These they were unwilling to destroy completely, but everything that was despised and weak they totally destroyed. 1 Samuel 15:1-9
Without even reading the rest of this story, we can sense the outcome. The Lord had commanded Saul (through Samuel) to destroy every living creature. Saul saved the stronger animals and the King of Agag. For whatever reason, God wanted the Amalekites completely annihilated essentially for what they did to Israel. (By the way, is this the future of all those who commit atrocious acts against Israel?)
The Lord spoke very clearly to Saul. Saul disobeyed. We may dismiss this and say how could Saul possibly disobey something so clear? I've said it a thousand times: when you point your finger at someone else, you have three pointing right back at you (your own). And often times it's not the large and grandiose things that we're disobedient in, but the little things. More so, it's not a hundred things we're disobedient in, but one or two, and as you read this, you know exactly what they are.
Ask God to give you strength to be obedient when it could cost you.