LtCmd.Lore
According to Hebrews 11:4 it was definitely the sacrifice that Yahweh didn't like. And according to Genesis 4:4-5 Cain didn't develope a bad attitude until AFTER the favoritism. Which is EXACTLY what TMS said.
Do you have any scriptures that would indicate that Cain was insincere BEFORE Yahweh rejected him?
Hebrews 11:4 makes it clear that faith was what led Abel do everything right. It says: "By faith Abel..." Whether it was the fact that he offered a blood sacrifice (which I don't think is the reason) or whether Cain just didn't offer his sacrifice with the right attitude, which God detected because duh! He's God and can read hearts and minds. Also, notice how God asked Cain that, "If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it." (Gen. 4:7) And 1 John 3:12 says that Cain's "works were wicked". And just to clarify before someone says - he was only wicked after he killed Abel - well, John says that because his works were wicked that's why he killed his brother Abel.
"If you do what is right..." implies Cain wasn't doing what was right and this doesn't seem to be referring to the act of offering the right things. It's followed by "will you not be accepted?" So Cain either had already some something bad, or was already bad-ish like be a mean brother, bad son, etc but not to the point of doing anything really serious/commit murder/sin gravely. If that was the case, as it seems to be from the text, God was already judging Cain and his attitude from past experiences. Remember, they were over 100 years old. Just because the Bible doesn't mention what they did before or how they behaved doesn't mean nothing happened between their births and the time when they made the offerings.
Justahuman - super but nonetheless