Your girlfriend was a clairvoyant who helped the police rid the town of murder. Every time someone was killed, your girlfriend was summoned to the crime scene where she would take steps back and forth over the body until a vision of the murder itself appeared in her mind. Sometimes she would spend 36 sleepless hours stepping back and forth over the dead body, looking like a child playing a hopscotch game or an obsessive compulsive performing a ritual, before the vision would come to her. But the visions always came. And while she might not have been able to ID the murderer every time, she always provided information that was useful to the investigation.
Naturally, the town's murderers realized they were pretty much out of business as long as your girlfriend was still alive. So they pooled their resources and killed her. Since your girlfriend was clairvoyant, it was necessary that all of the town's murderers create his or her own private plan for killing your girlfriend and that each murderer try to carry out that plan. This way, there would be so many people trying to kill her in different ways that when the foreboding visions of her impending murders started to pop into her head, they'd be so confusing and cluttered that she couldn't possibly take steps to avoid them all. In the end, she was killed by Squashfingers, the deformed loner who lives out by the railroad tracks. Squashfingers (his fingers are fat and swirly, so he kills people) killed her by rigging the international airport with dynamite so that when she stepped into the terminal on Thanksgiving weekend, it would blow.
Your girlfriend never would have died if her talent hadn't been utilized by the police department to solve all those murders. The town used her so that they could feel safe. And for their peace of mind, she paid with her life. It's time for the town to pay her back.
(Yes, you could just exact your revenge on the town's community of murderers, but they're all really dangerous. Stick with the townsfolk.)
Tonight, go to each house in town one by one and close all the windows and cover the cracks in the doors with towels. Then blow out the pilot light on each stove so that the houses will fill with gas. By dawn, everyone in town will have died from the gas. When federal investigators are summoned, they'll find the preprinted index cards that you'll have left on every house's coffee table. They'll read:
A Debt Was Paid
This Town Is Square
For those houses that don't have gas stoves, you're going to have to strangle everyone in their beds with your bare hands. If you happen upon some residents that are bigger than you and who will probably kill you first if you try to kill them, just let them live. Your girlfriend would have wanted you to stay alive. And even if you kill everyone but those buffest townsfolk who happen to own electric stoves, you'll still get your point across.
Happy Make The Town Pay For Your Girlfriend's Murder Day!