"And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen? 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." (Ge 4:6-7 AV)
Do you suppose Cain didn't believe the LORD before he killed his brother? We don't know of the conversations the LORD had with Cain prior to this but we can surmise that the same covenant sacrifice offered by Abel was also the expectation of Adam, Eve and Cain as well. Abel pleased the LORD with his offering and if this was the way presented to fallen men then it is clear Cain could have pleased the LORD as well. In fact the admonishment the LORD gave to Cain following his offering of groceries makes it very clear that Cain had a choice to make, a choice he was well capable of making. There are some who do not realize that fallen man by the grace of God, in this case the very presence of God and His Holy voice, is what enables otherwise cast down creatures to do the things pleasing to God. I emphasize this point again: Cain had a capable choice and the LORD admonished him to make the right choice. In continuing on his chosen path of unrighteousness and rebellion, he became a murderer and was held accountable for it. Now, the question can be asked "was the LORD sincere in His admonishment of Cain"? Did He truly desire Cain to repent and turn to Him? Unless the LORD is a hypocrite and God forbid any serious thought ever be forwarded as such, we can only subscribe to the truth that God is sincere in everything he presents to men. Because of the righteousness of God, the inability to be a hypocrite, we can know that Cain of his will, enabled by the Grace of God, had a free choice and he turned to his native self in seeing God's pleasure with his brother and the displeasure the LORD had with his own offerings. Cain rejected God … God did not reject Cain.