And the "heart only" thought process still doesn't take into account that Cain knew his sacrifice wasn't what God wanted...
It was definitely the sacrifice. If it was the heart only, why would God say why has your countenance fallen, if you do right will you not be accepted?
I think Cain knew it. God obviously spoke to them, He spoke directly to Cain. In the midst of Cain's sin no less. So yes God is not scared of our sin, it is us who hide from God. We see this in Adam & Eve's reaction.
Cain knew what he was doing was wrong. He probably knew the first sacrifice which was the animal that gave up it's skins to clothe Adam & Eve. And Cain decided he was going to give God what he thought God should want. His works. His effort. His best. Instead of God's. And God warned Cain and reasoned with him. But Cain ended up killing his brother.
He hated him in his heart. And without God's grace he flew off the rails.
C.