http://www.psywarrior.com/DeathCardsAce.html

We don?t know where the idea of an ace of spades representing death first originated. We do know that during the reign of ?Murder Incorporated? in the 1930s, at least two gangsters were assassinated and left with aces in their hands. One of the unidentified criminals with an ace of spades is depicted on the cover of the Burton B. Turkus book Murder Inc.: the Story of the Syndicate. Turkus also mentions Salvatore Marinzano, an early Cosa Nostra crime boss who was assassinated by a younger faction led by Lucky Luciano. When Marazano was killed, he had an Ace of Diamonds in his hand.
I found this on one of my go-to websites --> http://ace.mu.nu/