I Think

But Saul and the people spared Agag, and the best of the sheep, and of the oxen, and of the fatlings, and the lambs, and all that was good, and would not utterly destroy them: but every thing that was vile and refuse, that they destroyed utterly. 1 Samuel 15:9

Sin is disobeying God. It's that simple. Saul was told by God to destroy everything from the nation he conquered. Everything, including the good and the bad, the wanted and the unwanted. But Saul thought it would be good to save the sheep, oxen and lambs because he thought they would make good sacrifices. It was a very rational decision. He spared the animals that would make good offerings. He thought he was doing good. Most of us would think the same. I do what I think is good and my life is suddenly pleasing to God. But it's not that simple. 
  
   Saul was right; the animals would have made good sacrifices but God said to destroy everything. We hang on to things because we think they will be good. We think they'll help us. We try to make the best decision for ourselves based on our knowledge and rationality. But sometimes that's where we go wrong. Saul was completely rational. What he did made sense. But God said "no, I don't want you to do this." What may seem right in the eyes of men, is wrong in the eyes of God. God wasn't disappointed with Saul simply because he spared some animals. God was disappointed because Saul disobeyed Him. Saul ignored the voice of God and chose to go his own way against the calling of God. That was his sin. 
  
    If God is telling you to do something, do it. If He is saying not to do something, don't do it. He knows what He is doing and His knowledge supersedes ours greatly. It's not up to us to rationalize what God is doing, we'll only get confused. It is our job, however, to listen to His still small voice. If He says move, move. If he says stop, then stop. You don't have to rationalize it, just know that God said it. 

    Saul got into trouble because he acted according to what he thought, rather than according to the divine wisdom of God. He thought he was wiser than God. Remember that God's plan is perfect. It may not make sense, but it is perfect. In time, His will will be revealed unto you. Though it may seem difficult and you may not understand, follow Him. You don't need to understand, you just need to trust Him.