Genesis 4:6-10 meaning

God spoke to Cain and tried to lead him back to the right path. God was trying to encourage Cain to reject sin, but instead of taking God's counsel to heart, Cain resisted.

It seems there was more direct communication with God during this era. In this passage God speaks directly to Cain, and tells Cain that sin is seeking to control him and that he has no reason to be mad at God. What Cain needs to do is straighten up and do right, then he won't have a reason to be mad. This is another indication of the amazing gift God gave to humanity, the freedom to choose. That is one of the primary ways in which we are made in His image (Genesis 1:27).

The book of 1 John reveals more about Cain's thoughts and intention,

"Cain, who was of the evil one and slew his brother. And for what reason did he slay him? Because his deeds were evil, and his brother's were righteous"
(1 John 3:12).

Cain killed Abel because of he was jealous of Abel and likely because he (Cain) had not given God the best of what he had to offer. 

In verse 7, God warned Cain that temptation was a fierce beast crouching at the door of his life. Here, sin is personified as an animal. Cain carried a grudge or harbored a bitter feeling in his heart. God told Cain that he must master it, but Cain was unwilling to get right with God. The Hebrew phrase about desire and mastery is the same as in Genesis 3:16. God makes it clear to Cain that sin is trying to devour him, and that it is his choice whether to give in to it or to master it. Cain will decide. And unfortunately he decided to give in to sin.

Cain murdered his own brother out of envy and hatred. This wasn't a sudden decision, it began with his anger. God intervened to warn Cain, but Cain ignored the warning. That means the murder was premeditated. In spite of God's warning and promise, Cain murdered his brother, and then lied to God, which is futile, since God knows all things. The first physical human death in the bible was a murder. Cain refused to show respect to God and instead, he showed his selfish defiance.

When Cain lied to God trying to cover up his wicked deeds, God asked questions. Just as He did in Genesis chapter three with Adam and Eve, God was giving Cain the opportunity to tell the truth, and confess his sin.

Cain and Abel, actual blood brothers, were separate in spirit. They both knew God, but one was righteous and one wicked.