Cain may have thought he was getting away with something, but God sees everything. God knows the truth. Since there is no secret sin it is foolish to make excuses. Adam blamed Eve, Eve blamed the serpent, Cain blamed Abel. Every one of those folks would have been better off seeking the mercy of God.
Unconfessed wrong creates a cancerous condition in our soul.
When we don't acknowledge and repent of our wrong, we leave a nail that the Devil can use. He will and does. As a result of Cain's act of premeditated murder God pronounces a sentence on Cain, Now you are under a curse and driven from the ground, which open its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.