Complete Dictionary of Dreams
by Dr. Mıchael LennoxA massacre is an event of great magnitude by virtue of the number of deaths caused by a single force. Any death is a powerful symbol of transformation and change, as it always portends a rebirth of some kind.
A mass death can be interpreted as any death would be—change so great that it can only be understood as the passing of something old to make way for the rebirth of something new. Look to your life for evidence of some change or shift that feels inordinately large in scale and you will probably find the stimulus for this dream. Since people in your dreams represent parts of your personality, the loss of large numbers could indicate that many old thoughts or patterns are being left behind.