"the Mark of Cain"

The Mark of Cain is referred to in the Biblical Book of Genesis.  Cain is the eldest son of Adam and Eve, and murders his brother Abel in a pique of jealousy.  This greatly angers God, who curses Cain, saying that he will no longer be able to till the land and grow crops, but must wander the earth alone.  Cain says he will surely be killed if this happens; God responds that this is not so, and that "if anyone kills Cain, he will suffer seven times over".  He then sets a mark upon him to signify this.  The actual definition of this 'mark' has been debated for centuries, but in common parlance it is used to describe a person carrying guilt, often because they have killed someone.  A famous example of its use was in the eve-of-battle speech by Colonel Tim Collins in Iraq in 2003.  He warned his troops, "I know of men who have taken life needlessly in other conflicts; I can assure you that they live with the Mark of Cain upon them".

Listen to the song 'Lullaby for Cain' by Sinead O'Connor