Massacre Girl when several creatures die at once
It depends on whether or not Massacre Girl entered the battlefield this turn.



Massacre Girl's ability reads:


  When Massacre Girl enters the battlefield, each other creature gets -1/-1 until end of turn. Whenever a creature dies this turn, each creature other than Massacre Girl gets -1/-1 until end of turn.


As you can see, both sentences are part of the same ability. When this ability resolves, it creates a delayed trigger for the rest of the turn, which will trigger whenever a creature dies.

If Massacre Girl entered the battlefield this turn, even if her ability didn't kill any other creatures itself, it'll have created the delayed trigger, and that will trigger whenever a creature dies (including Massacre Girl herself). Otherwise, the delayed trigger doesn't exist and will, therefore, not trigger.