In reality, it was a sarcastic, lying retort by Cain to God denying his murder of Able...
Then the Lord said to Cain, "Where is Abel your brother?" He said, "I do not know. Am I my brothers keeper?" - Genesis 4:9
So, it is a murderer's lie, by a murderer denying his cold-blooded murder of his own brother and then trying to cover it up.
Somehow, it fits the attitudes of these people to a tee even if they don't know it.
Hmm, it’s almost as if Obama picked that slogan on purpose.