3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord.
Somewhat nonspecific as to the quality of the offering, yet this is not the main issue as we already know God has set the standard for sacrifice as one requiring innocent blood being shed.
4 And Abel, he also brought of the firstlings of his flock and of the fat (choice or best) thereof. And the Lord had respect (to inspect, consider) unto Abel and to his offering:
Let’s recall to our minds that at this time man was to only eat plants….. not animals. Therefore the purpose for animals in this era was NOT for meat, rather it was first for enjoyment; then for appointed sacrifice and clothing. The animals Abel was producing could have served no other purpose than as a service to God and for the wool or skins for clothing and perhaps tents.
So we could further combine these two purposes for the animals under one title: covering. Do you see this? The Sheep, the Lamb, was to provide covering [clothing] for man’s physical nakedness, and it was also to provide covering [its own innocent blood] for man’s spiritual nakedness, his sin. But it was not meant for his nourishment.
5 But unto Cain and to his offering he had not respect. And Cain was very wroth (be angry, burn, be displeased), and his countenance (facial expression) fell.
6 And the Lord said unto Cain, Why art thou wroth? and why is thy countenance fallen?
Matthew 23:32-35 “Fill ye up then the measure of your fathers. Ye serpents, ye generation [G1081 γέννημα gennema (ǰen’-nee-ma) n. offspring.] of vipers, how can ye escape the damnation of hell? Wherefore, behold, I send unto you prophets, and wise men, and scribes: and some of them ye shall kill and crucify; and some of them shall ye scourge in your synagogues, and persecute them from city to city: That upon you may come all the righteous blood shed upon the earth, from the blood of righteous Abel unto the blood of Zacharias son of Barachias, whom ye slew between the temple and the altar.”
G3789 ὄφις ophis (o’-fis) n.1. a snake. 2. (figuratively, of sly cunning) an artfully malicious person.3. (especially) Satan.)
7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
God reminds Cain here that he has the opportunity to follow the instruction of God and bring the proper sacrifice and all will be well and good in His sight. In the very same manner as Adam and Eve in the garden, once Cain had it in his mind that there were other options the Talebearer, or Satan, would be there to influence his behavior and enslave him to the chains of sin.