Proof of Free Will Based on the Cain and Abel Account
Genesis 4:7 (DRA)
If thou do well, shalt thou not receive? but if ill, shall not sin forthwith be present at the door? but the lust thereof shall be under thee, and thou shalt have dominion over it
These were the words spoken by the Lord to Cain before he slew his brother in his envious rage. It is basically an encouragement that if he does well, then he will have the Lord's favor. Not only that, but he blesses Cain that he shall have dominion over sin (notice the use of the word "shall", which entails that this is a certainty).
We all know what happens afterwards. Instead of examining why his sacrifice was deemed less worthy, he goes out to commit this heinous act instead. A story like this will hit home, because nearly all of us have the moments where we face our faults. Often it is a good opportunity for personal growth and "to have dominion" over that fault, or to blame everybody but yourself for it. Not only that, but to also inflict pain because you didn't get the desired outcome from your doings. This doesn't mention the muddy water where one may be a victim of circumstance where bad things happen that are no fault of their own. Though it is a sin for someone like this to lash out, it is certainly more forgivable.
In Cain's account, there are no excuses. He witnessed the glory of God right in front of him (something that none of us will ever get to see in our earthly lives), gives him this wonderful blessing, yet still commits this terrible evil. He was meant to take the Godly path, yet he chose freely towards the opposite. If there is a message that the Cain and Abel story was meant to convey, I believe it is that everyone of us has the choice on which path we take in life, that it is certainly not predetermined.
Addendum: Why was Abel's sacrifice accepted but not Cain's?
Genesis 4:3-4 (DRA) 3 And it came to pass after many days, that Cain offered, of the fruits of the earth, gifts to the Lord.
4 Abel also offered of the firstlings of his flock, and of their fat: and the Lord had respect to Abel, and to his offerings.
After reading a variety of commentaries, there were two scenarios which made the most sense to me. Notice not what's included in Cain's offering, but rather what is omitted. In Abel's offering, it mentions "firstlings of his flock", while in Cain's it merely states "the fruits of the earth". Meaning Abel gathered the best he could offer, while Cain did not. The other explanation has to do with Cain's proud nature. Abel may have had a pure intent as he offered his sacrifice out of a selfless love for the Lord, while Cain only offered his sacrifice so the Lord can see him in a high favor.