Adapted from Creation Answer Book by Hank Hanegraaff 

First, we must not suppose that Abel’s offering was accepted because it was a blood sacrifice and Cain’s rejected because it was bloodless. Each gave according to his vocation, but Abel gave the firstborn, the fattest of his flock, while Cain gave fruit that was not the firstfruits. As such, at issue was a disposition of the heart.

Furthermore, Cain’s disposition is evident in his angry and downcast countenance. Sin reigned in him with such vulgar vengeance that he embodied the character and craftiness of his father the devil—“a murderer from the beginning” (John 8:44). Cain invited an unsuspecting Abel to the field and there murdered him with malice and forethought.

Finally, Cain’s evil disposition is evidenced in the complaint regarding the severity of his punishment. He was bothered not by the content of his sin but by its consequences. It was not penitence but punishment that caused Cain to plead for protection from Adam’s posterity. Cain’s offering in Genesis highlights the reality that man looks on the outward appearance, but the eyes of God pierce externalities and perceive the intent of the heart.

The Lord looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
Genesis 4:4–5