Changeling by Ray Bradbury - Leonard–or the thing that had been Leonard–must be destroyed!

By eight o'clock she had placed the long cigarettes and the wine crystals and the silver bucket of thin shaved ice packed around the green bottle. She stood looking at the room, each picture neat, ashtrays conveniently disposed. She plumped a lounge pillow and stepped back, her eyes squinting. Then she hurried into the bathroom and returned with the strychnine bottle, which she laid under a magazine on an end table. She had already hidden a hammer and an ice pick.

She was ready.