Cain Was Very Angry

Genesis 4:3-7 NIV
In the course of time Cain brought some of the fruits of the soil as an offering to the Lord. 4 And Abel also brought an offering—fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering, 5 but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
6 Then the Lord said to Cain, “Why are you angry? Why is your face downcast? 7 If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must rule over it.”

Why did God approve of Abel's sacrifice and not of Cain's. God is already teaching us the need for shedding of blood for our sin. Though death had entered the world through disobedience, God was pointing the way back to the way of life, preparing us for the sacrifice of His Son.
In this passage we see God appeal to Cain to do the right thing. Cain knows, but refuses to do so. He goes even further in his opposition of God by spilling the blood of his brother. He is cursed as a restless wanderer and travels east, there to remain in Satan's grip.
It foreshadows these latter days as the descendants of Cain seek to destroy Israel.