God said "ye shall not eat of the tree". The sin from the outset lay in just eating of the tree let alone what the tree might produce in us.
To me, human autonomy, "ye shall be like gods" was the motivating factor. There is in humanity the innate desire to be like his creator but this snake-invite had some twists to it.
Tree of the knowledge of good and evil. Knowing right, wrong, good, evil...all sounds like a tree where one goes to obtain a knowledge of these things. A tree of morality.
Yet, forbidden of God. Why? Because through hindsight learning, we found out what that serpent had in mind.
First, you had to be immoral=disobey God before you partake of a tree giving knowledge of morality.
Secondly, You had to leave God out of the equation like Adam and Eve did. They didn't call out to God, didn't pray, didn't say "hey, lets find out why God's holding back, lets meet up again with him?"
Nope, just straight to the tree autonomously, and of course Adam was in arms-reach of Eve and partook himself.
What happened? They understood in a measure what happened, a sense of guilt, hiding, their human autonomy at first innocent, now guilt-laden.
The way I see it, sin is leaving God out of our lives, seeking after wisdom, knowledge, good, evil, right and wrong and determining by our own understanding how we should thus live.
The knowledge of good and evil is to come by way of the revelation of God, not by self seeking what appears to be good to eat and then partaking of it.
As to Cain's offering, God gave both seed to the sower and bread to the eater and all the beasts of the field.
Yet, Abel's was better...and I think Havok nailed it there.
Faith...but I would not rule out the thought that without blood there is no remission for sin. Grain offerings are not sin-offerings per-se.