Serial killer

A serial killer, or mass murderer, was an individual who murdered several individuals in a similar manner, choosing their victims based upon a common theme. It was sometimes caused by psychosis.

One famous serial killer 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. 

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.