Grace abounds to us (Genesis 4)

And in the process of time it came to pass that Cain brought an offering of the fruit of the ground to the Lord.

Abel also brought of the firstborn of his flock and of their fat.

And the Lord respected Abel and his offering, but He did not respect Cain and his offering. And Cain was very angry, and his countenance fell.

So the Lord said to Cain, “Why are you angry? And why has your countenance fallen? If you do well, will you not be accepted? And if you do not do well, sin lies at the door. And its desire is for you, but you should rule over it.”

And Cain said to the Lord, “My punishment is greater than I can bear! Surely You have driven me out this day from the face of the ground;

I shall be hidden from Your face; I shall be a fugitive and a vagabond on the earth, and it will happen that anyone who finds me will kill me.”
And the Lord said to him, “Therefore, whoever kills Cain, vengeance shall be taken on him sevenfold.” 

And the Lord set a mark on Cain, lest anyone finding him should kill him.

The Father heart of the Father revealed again.

Notice the Father did not reject the offering but He knew it was not his best. 

Cain had a second chance to do better next time.

Instead he chose to let someone else take the blame (remember Adam and Eve) and killed his brother.

When he cried the Father changed his punishment and protected him.

His grace abounds to us.

I will be their God