@SuperSnekFriend @BowsacNoodle I'm glad you mentioned this! It's an old argument I still see trotted out sometimes. I agree it had nothing to do with what was offered or even "how" it was offered, procedurally speaking, but the state of Cain's heart.

Cain's heart and deeds were evil and the entire story of Cain is one of a man who doesn't seek repentance. God gently chides him for the state of his heart and even tells Can his sacrifice will be accepted if he subdues the sin crouching outside. (As a side note, I've always loved the crouching imagery, as it brings to mind a beast ready to pounce, an active participant, demonic in origin [1 Peter 5:8]) Cain refuses to subdue his passions, gives in to them, murders his brother, lies to God. Then, even after God places a curse on him, Cain shows no sign of remorse or sorrow for anyone other than himself! He is worried only that people will take vengeance on him! Unrepentant and sinful to the end.

Thus, while Adam brings death into the world, Cain is often visualized as the father of sin, due to his unrepentant heart, as seen in 1 John 3 or Jude.