Monster

She said she killed them because they were monsters. Mary Blanc walked into the party with a loaded shotgun. In the blink of an eye she blew two people away. She wanted to kill more, but was stopped by her best friend, Angela Warner, and the police. The next day, when Angela visits Mary at the jail, she asks why she did it. Mary responds, "Because they were no longer human." Angela thinks she's crazy. At first.
Until she probes deeper into Mary's claims and discovers a horror so unimaginable that she thinks she is going crazy. She even gets to the point where she wonders if she should have let Mary keep on killing. While there was still time to stop the thing from spreading. The very old thing. The hungry thing.