Judge Wargrave explains this symbol in his secret letter, sealed in a bottle. He writes that the mark on his forehead is one of the three clues that could help explain the nearly unsolvable murder he committed on Soldier's Island. The bullet wound on his forehead mirrors the mark that Cain received from God after he murdered his brother Able, thereby committing the first murder in the Bible. This mark shows that even Wargrave admits his own evil. As his letter shows, he knows that he brought these guests to Soldier's Island not only to seek justice for their unpunished murders, but also out of his own bloodthirsty desire to kill. The mark symbolizes his sinfulness, and by extension the sinfulness and guilt of all of the people on the island.