Let's see what the Bible says (note the underlined words):
"6Then the Lord said to
Cain, “Why are you angry? Why is your face downcast?
7If you do what is right, will you not be accepted?"-Genesis 4:6
"
Cain,
who was of the evil one and slew his brother. And for what reason did he slay him? Because
his deeds were evil, and his brother’s were righteous. "-1 John 3:12
It doesn't say anything about what they were thinking or feeling. The Bible says it was about what they
did. Cain did not bring the right sacrifice. Abel did. Abel obeyed the Lord, Cain did not.