What were the three clues mentioned at end of Agatha Christie's And There Were None?

Question

In the end of the book it is said that the culprit believes, even though the crimes commited were perfect and police will be unable to solve it, there are three clues left that can help identify the Killer.What are these three clues?

Answer 1

The first clue was the bullet through the head of the first killer, symbolizing the "mark of cain", thus revealing that Cain was the first murderer. The context behind Christie's selection of this clue can be found in Genesis 4:15.

The second clue reveals that the actual killer was the only one not truly guilty of murder. His crime was sending Seton into the gallows, although he did not have conclusive evidence. The killer however, justifies his actions by exposing that Seton was guilty of several other misdemeanors, and nevertheless deserved to hang.

The third clue ties back to the poem presented at the beginning of the book that states, "A red herring swallowed one and then there were three". The "red herring is a concept that represents a trick or distraction, and this implies that somehow Armstrong was implicated in the plot.