And Samuel said,"Has the Lord as great delight
In offerings and sacrifices,
As in obeying the voice of the Lord?
Behold, to obey is better than sacrifice
Than to listen in the fat of rams.
For rebellion is as the sin of divination,
And presumption is as iniquity and idolatry.
Because you have rejected the word of the Lord,
He has also rejected you from being king."
Saul was supposed to kill all the Amelikites, but he didn't. So Samuel was telling Saul that since he has disobeyed, God was rejecting him from being king.
1st Samuel 15:22-23