Brother’s Keeper

“Now Cain said to his brother Abel, “Let’s go out to the field.” While they were in the field, Cain attacked his brother Abel and killed him.

Then the LORD said to Cain, “Where is your brother Abel?”
“I don’t know,” he replied. “Am I my brother’s keeper?”” (Genesis 4:8-9)
How did Cain miss the fact that he was his brother’s keeper? I’m not sure. ///