"The LORD said to Cain, 'Where is your brother Abel?' 'I don't know,' he replied. 'Am I my brother's keeper?' — Genesis 4:9
Let's consider the case of the original murderer: Cain, the son of Adam. Cain admitted nothing. Cain confessed nothing. Even when given the specific opportunity, even when confronted by God himself with the evidence of his brother Abel's blood on the ground, Cain admitted nothing.
"Where is your brother?" was God's simple question, and Cain answered that he had no responsibility to keep his brother from harm. This was the response of the man who had just murdered that brother.
To the God who knows all, Cain lied: "I don't know where he is." But of course Cain knew where he had left Abel dead.
And did you notice that Cain seems to have thought he should still have God's protection? Without confessing his sin, or even being sorry for it, Cain thought it was unfair that he should be a cursed wanderer. He complained to God, "I will be hidden from your presence." But even that wasn't true, as God made clear to him.
Standing in that blood-stained field, claiming to be without sin, Cain deceived no one but himself.
Are we deceiving ourselves? Are we repentant? Are we our brother's keeper?