I'm going to put this here. While I have seemed to improve on scenarios involving the snuffing of individual girls or a small group, I suck at large scale massacre scenarios (like girl hunts or school shootings). So here's two ideas that I hope someone can execute better.

Princess Hunt

In a fictional kingdom the males in the line of succession must find wives at the age of fifteen. The tradition is to gather 50-100 girls from twelve to fifteen years old are taken into a secluded area (island, walled off forest, abandoned town, etc) and allow to prince to hunt and kill as many as possible over the course of six hours. The prince selects his wife from among the survivors.

My attempts to write this had a kingdom with modern technology, but any a pre-industrial or futuristic society would be interesting as well. In the modern setting the event is televised, but you may choose to have it that way or not.

Babe Hunt

There's a place called Babe Safari, where hot and scantily clad women walk around the wilderness. Once a year an event is hosted where hunters are allowed to rape, shoot, and even eat as many of the woman as they can during the allotted time.