The importance of how we worship God and how we feel about Him...
The best analogy is the one right from the beginning.
Genesis 4:3-7 is a picture of Cain and Abel's sacrifice. God see's the heart. He knows us and why we do what we do.
These two sacrifices were different.
One was done his own way (Cain) and one was done according to what God wanted in obedience (Abel).
Genesis does not explain how the practice of sacrificial worship began but it is clear that Adam and Eve's two son understood the custom.
Their sacrifices were acts of worship.
It was the deficiency in Cain's heart, not in the actual offering. Abel's offering was "more excellent" than Cain's because of his faith in the Lord.
In vs 8 Cain talked with Abel, trying to understand what God said. Cain was disturbed and wanted to justify his actions. But instead he got mad after talking to Abel and when they were alone, working in the field, Cain attacked Abel and killed him.
In vs 9 The Lord asked Cain where is Abel, "your brother"? Cain lied to God.
In vs 10 The Lord says that the voice of your brother's innocent blood is crying out to the Lord from the ground (for justice).
God knew the hearts. God allowed each to choose. And yet we learn that God also didn't put Cain to death.
The Lord cursed the ground because Cain murdered his brother Abel.
In vs 13-14 Cain says to the Lord my punishment is greater than I can bear and he feared for his life.
But God says in vs 15 that anyone that kills Cain, a sevenfold vengeance [that is, punishment seven times worse] shall be taken on him, by God Himself.
If you think about it you begin to understand God's Mercy. Here is one of God's children who kills another of God's children, wrongfully and horribly and God now protects Cain.
Another great example of God's love and mercy for us.
If God is so quick to strike us down, as many try to say, then why did He protect Cain. We really don't understand God's Love, fully.