A massacre in a dream is not a forecast; it is traditionally read as shock and helplessness at a scale too large to take in, often stirred by news or old trauma.
Where in your waking life have you met something like this lately?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.