In reading the account of Saul, it's easy to wonder what his big mistake was.  Why was he suddenly unworthy of being king over Israel simply because he offered sacrifices to God before going in to battle?  After all, Samuel didn't come when he said he would, and Saul had no idea what might have happened.  The key to understanding this seemingly extreme punishment lies in what Samuel says in 13:14, "The LORD has sought out a man after his own heart…"  The problem was that Saul wasn't a man after God's heart.  To the contrary, he was concerned with victory for his name's sake.  He wasn't so concerned about honoring the Lord as honoring himself, and he fell into the same sin of believing that the Lord could be controlled by simply offer the right sacrifices and saying the right things – in the process, though, he neglected to respect God's holiness.  The offering of sacrifices was the role of the priests, and Saul, rather than respecting what God had instructed to the people, tried to manipulate the circumstance in his favor.  Any time we try to manipulate God we step outside of respecting His holiness.  God is not a god to be manipulated.