Cain

Genesis 4:6 "Then the Lord said to Cain, 'Why are you angry? Why is your face downcast? 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 master it."

Silence. Cain did not respond to God's advice. Instead, he killed his brother.

"I hate you, Abel!"
Thought brutish Cain.
From that one thought
pure Abel was slain.

"My offering was just as fine as his.
I should find favor,
I've been ignored.
Now his death will be my bliss!"

So God made Cain aware of the consequences of this sin. He would be banished, yet when Cain expressed his fear of death, God eased his worry, promising vengeance on any who might harm Cain. Why? Mercy, perhaps. Perhaps God hoped in Cain's reform. Perhaps that is why He extends mercy to us all.