Atheists love to take this verse and point out that God is arbitrary and unjust.
The truth is, He delights in mercy (grace) and not just sacrifice:
"For I desired mercy, and not sacrifice; and the knowledge of God more than burnt offerings." (Hosea 6: 6)
Jesus would rebuke the Pharisees of His time with the same verse:
"But go ye and learn what that meaneth, I will have mercy, and not sacrifice: for I am not come to call the righteous, but sinners to repentance." (Matthew 9: 13)
So, anyone who makes a fuss about God sparing Cain should recall the above revelation about God's true nature.
Now, what about Abel?
Yes, he offered the proper sacrifice, a lamb, a picture of the Lamb of God who takes away the sin of the world (John 3: 27).
Yet there was something Abel did not have which Noah, Abraham, and we today have:
"But with thee will I establish my covenant; and thou shalt come into the ark, thou, and thy sons, and thy wife, and thy sons' wives with thee." (Genesis 6: 18)
"And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee." (Genesis 17: 7)
Consider yourself today, in Christ:
"1There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit." (Romans 8: 1)
"31What shall we then say to these things? If God be for us, who can be against us? 32He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things? " (Romans 8: 31-32)
"10For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
11And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.
12For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more." (Hebrews 8: 10-12)
Because of Jesus, we have a better covenant, under which we can rest assured of God's eternal, loving protection.