1 Samuel 15:22 (NKJV)
So Samuel said: "Has the Lord as great delight in burnt offerings and sacrifices, As in obeying the voice of the Lord ? Behold, to obey is better than sacrifice, And to heed than the fat of rams.
King Saul had been given instructions from God through Samuel to wipe out the Amalekites and all of their livestock too. Saul decided to keep the cattle and sheep. He figured that it would be such a waste to destroy them all. After all, he could use them to make sacrifices to God. Seems like a pretty good rationalization. But it was a direct disobedience of God's command.
God instituted the sacrificial system to teach us about the ultimate sacrifice that He would provide through Jesus Christ. Sacrifice must cost something. Saul's planned sacrifice was not a sacrifice at all, he was attempting to gain material value through deception.
We are all pretty good at this type of rationalization, calling wrong things good things to get what we want. The problem is, we don't even see it for what it is.
Proverbs 15:8-9 (NKJV)
The sacrifice of the wicked is an abomination to the Lord, But the prayer of the upright is His delight.
The way of the wicked is an abomination to the Lord, But He loves him who follows righteousness.
Psalm 139:23-24 (NKJV)
Search me, O God, and know my heart; Try me, and know my anxieties; And see if there is any wicked way in me, And lead me in the way everlasting.