Really the first mention of violence - from the wallpaper. "Like a bad dream" - Well she is in a bad dream, except that it's reality. You can escape a bad dream, but you can't escape reality, or this hideous, unreliable, infuriating, and torturous wallpaper. She can't stop indulging in the wallpaper, immersing herself in the violence of her surroundings.
In the next paragraph she likens an arabesque to a fungus, which is nature, except not the pretty romantic nature that is stereotypically healing. In the beginning she looked out the window to see nature (roses, lanes, trees) and now she is talking about fungus and toadstools - nature, but a distorted nature, like her supposed mental state. Towards the end of the story she says, "For outside you have to creep on the ground, and everything is green instead of yellow." Yellow is dying nature, she is slowly dying. She recognizes that she isn't living anymore, and if she isn't yet dead then she must be dying.