Since there probably won't be a page for the character, I am creating this blog so people can post their theories about the killers identity.
Here are mine:
Poison Ivy: Since she used her powers to take over the world once, it wouldn't be hard for her to make the residents of Sanctuary kill each other, trick Booster and Harley into thinking they did it and fake her death. Also, her corpse was the only one of the Sanctuary victims that Superman did not see when he was investigating the building.
One of the Robots: There's also the chance that one of the Sanctuary robot went crazy and killed everyone. It wouldn't be the first robot based on a superhero mindset that went crazy and killed people.