“Why are you angry,” said the LORD to Cain, “and why has your countenance fallen?
6. And the Lord said unto Cain . God now proceeds against Cain himself, and cites him to His tribunal, that the wretched man may understand that his rage can profit him nothing. He wishes honor to be given him for his sacrifices; but because he does not obtain it, he is furiously angry.
God is here reasoning with Cain, to convince him of the sin and folly of his anger and discontent, and to bring him into a good temper again, that further mischief might be prevented. It is an instance of God's patience and condescending goodness that he would deal thus tenderly with so bad a man, in so bad an affair.
Commenting on Genesis 4:6-7
And the Lord said unto Cain, why art thou wroth? and why is thy countenance fallen?.... Which was said not as being ignorant of his wrath and resentment, but to bring him to a conviction of his sin or sins, which were the cause of God's rejecting his sacrifice, and to repentance and amendment; and to show him that he had no cause to be...