So contrary to our modern Christian sensibility, we get lost in the slaughter and the point of the first reading can get lost on us.
Saul, after becoming king, goes on a killing rampage. God through Samuel says fine, but if you are going to kill you must kill everything and everybody. Saul likes the idea of killing everybody, thereby getting rid of all threats to his power, but decides not to kill everything. The best of the animals he allows to live, the spoils of war. This was precisely what God had forbidden. War may be necessary, but profiting from it was a sin.
When confronted by Samuel with his disobedience, he does what we humans all too often do. Instead of simply confessing his sin, he invents an explanation that sounds not only good but pious.
But from the spoil the men took sheep and oxen, the best of what had been banned, to sacrifice to the LORD their God in Gilgal.
Notice the multiple layers of dishonesty. First of all he distances himself from the act. He claims he had nothing to do with it "the men" did it. Then, like the cherry on top of the cupcake, he drags God into it, claiming they kept the best animals to sacrifice to God.
God's response is to declare that Saul is no longer fit to be king.
One is left to wonder how different God's response might have been if Saul had simply said, "I did it. I messed up. It's my fault."