More info please? Please re-post the question with your age, weight, allergies, medical conditions, which mouthwash and what are the ingredients? There is no simple answer to your question. Basically most brand name mouthwashes are safe and some accidental ingestion is not a major problem nor will it likely cause death. Even salt can cause death if taken in large enough quantities over a short period of time.
Mouthwash. I am not familiar with the notion that mouthwash can kill you. However, most mouthwashes have some amount of alcohol in them, so if you are allergic to alcohol, or have a hypersensitivity to alcohol or any of the ingredients in a particular mouthwash, i would consult with your md or an allergist to see if you need to be concerned or should avoid using them.