A mass murderer was an individual who committed mass murder, that is, murder of several individuals during a particular time frame or manner, choosing their victims based upon a common theme. Such a murderer could be known as a serial killer. Their actions were sometimes caused by psychosis.
One famous mass murderer in Earth's past was "Jack the Ripper" (aka 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")
The Vulcan Chu'lak killed several inhabitants of Deep Space 9 in 2375, choosing his victims based upon them having photographs in their quarters which displayed emotion. (DS9: "Field of Fire")
In approximately 3074, The Doctor's EMH backup module was upset at how the crew of the USS Voyager was depicted in the Kyrian holoprogram The Voyager Encounter. Portrayed as monsters, Captain Kathryn Janeway was a cold-blooded killer, the crew was a gang of thugs, and The Doctor was a mass murderer. (VOY: "Living Witness")