How did God show He was displeased with Cain, and did God speak to him personally?

Nothing either good or bad is recorded of either of these men, Cain and Abel, up to the time of their presentation of their respective offerings before the Lord. It was natural and proper that each should present an offering representing his own toil. The Lord's acceptance of Abel's offering and failure to accept Cain's should not be considered as a slight against Cain, but rather as a lesson of instruction. God was teaching them what kind of offering would be most acceptable and pleasing. The reason God accepted Abel's offering was because God wanted all offerings to recognize original sin. Abel’s offering was the sacrifice of a sheep. This would symbolize the necessity for the great sin-offering, the sacrifice of the life of Jesus. Jesus was symbolized as the “Lamb slain before the foundation of the earth” in Revelation 13:8. Thus early did God begin the lesson of instruction emphasized in Hebrews 9:22, “without the shedding of blood there is no remission of sins”.

God did not leave Cain to himself, but recognizing inexperience, the Lord admonished him with the question, “Why are you angry, and why has your face fallen? If you do well, will you not be accepted? And if you do not well, sin is crouching at the door. Its desire is for you, but you must rule over it.”

Cain should at once have appealed to the Lord to learn how to prepare a pleasing sacrifice. The inference that he was not now doing well was a reprimand; and the suggestion that sin was “crouching at his door,” should have been a warning. Nothing is intimated of a wrong heart condition prior to this sacrifice, and the sacrifice itself was not wrong—it was merely that Cain was ignorant. The wrong began when he became angry instead of learning the lesson. Sin was now crouching at his door like a wild beast, ready to spring upon him and devour him. And, alas, he failed. Cain became a murderer. It was the murderous spirit of Satan that filled his heart. Jesus identified Satan as a “murderer from the beginning” (John 8:44) and that angry disposition overcame Cain.

Did God speak to Cain personally? God did speak to Cain but by what means He spoke to Cain is not specified. The Bible records many instances where God used angels to speak for Him. That is probably how God communicated with Cain. For example, in Genesis 18:13, it sounds as if God, Himself, is speaking to Abraham. Yet, when we read the whole story in Genesis 18, God was delivering His message through three angels.