When the Bible says that God repents, it means that he expresses a different attitude about something than he expressed before, not because any turn of events was unexpected, but because the turn of events makes a different attitude more fitting to express now than it would have been before. The result of his night of prayer is a firm resolve to do what God says. But nothing Saul says will work now. I see at least five reasons in this story why God hates disobedience and takes pleasure in obedience. I'll mention them in the order from least to most serious, as it seems to me.
Disobedience shows a misplacement of fear.
Because he feared the people instead of God. He feared the human consequences of obedience more than he feared the divine consequences of sin. He feared the displeasure of the people more than the displeasure of God. And that is a great insult to God. He said they wanted to sacrifice these to the Lord in Gilgal.