Am I my brother's keeper?

"Am I my brother's keeper?" is a saying from the Bible. It comes from the story of Cain and Abel: After Cain murdered his brother Abel, God asked him where his brother was, and Cain answered: "I know not; am I my brothers's keeper?" These words came into the English language through William Tyndale and his translation of the Bible into English.
The story is in Judaism and Christianity. It shows not only Cain's wrongness in not looking after his brother's well-being, but his hiding of the terrible crime of fratricide.