Longman Dictionary of Contemporary English
Wiktionary
n. Someone who commits a mass murder.
WordNet
n. a person who is responsible for the deaths of many victims in a single incident
Usage examples of "mass murderer".
An attractive female journalist named Sandy Fawkes met Knowles in Atlanta, and spent several days with him without suspecting that he was a mass murderer.
In this world, you got a two-hundred-year-old mass murderer spouting orders at you before your eyes were all the way open.
They start to open up about each other -- but Zeth reveals a little too much, and Fen realizes to her horror that Zeth is really Kyp Durron - the most infamous mass murderer since Palpatine.
Of course, the siege of Leningrad was one of the great battles, one of the turning points in human history, the army of one mass murderer throwing back the army of a fellow mass murderer.