A mass murderer, was an individual who committed mass murder, that is, murder of several individuals during a particular time frame or manner, and choosing their victims based upon a common theme. It was sometimes caused by psychosis.
One famous mass murderer in Earth's past was "Jack the Ripper" (a.k.a Redjac), who was known for killing women. Under several other aliases, it killed on multiple planets as well. After doing so, it fed on their fear. These murders were not considered to be related, nor was its motive known to investigators until 2267, during the inquiry into murders believed to have been committed by Montgomery Scott during his medical leave on Argelius II that year. (TOS: "Wolf in the Fold")