The LORD God gave Cain the opportunity to confess to the crime of taking his brother’s life.  But he did not, he became instantly defensive:
(NASB) Genesis 4:9 “Then the LORD said to Cain, ‘Where is Abel your brother?’ And he said, ‘I do not know.  Am I my brother’s keeper?’”

Cain’s anger had been given full control over his actions.  So much so that he took the life of his brother; so much so that he couldn’t bring himself to confess or repent. Cain chose anger over God.  Cain is an example of wrong choices.

(NASB) 1 John 3:11-12 “For this is the message which you have heard from the beginning, that we should love one another;
12 not as Cain, who was of the evil one and slew his brother.  And for what reason did he slay him?  Because his deeds were evil, and his bother’s were righteous.”

There are always consequences to our sins…not only are we affected but the natural world is also involved:
(NASB) Genesis 4:10-12 “He (God) said, ‘What have you done?  The voice of your brother’s blood is crying to Me from the ground.
11 ‘Now you are cursed from the ground, which has opened its mouth to receive your brother’s blood from your hand.
12 ‘When you cultivate the ground, it will no longer yield its strength to you; you will be a vagrant and a wanderer on the earth.’ ”

How God must have given the pronouncement of Cain’s punishment with sorrow and grief.  Cain had refused to deal with his anger (See Genesis 4:3-6) and it led to the murder of his brother.  Crime and punishment, the personal punishment Cain received and with which he lived with the rest of his life.
The LORD God does not ‘sweep sins under a rug’ nor does He hope they go away.  He brings sins out into the open and deals with them.  Love does that.

God’s love does require us to answer the question: “Am I my brother’s keeper?”