First of all, I would like to inform you that a Living Will and Mercy Killing are not the same. They are different.
Living Will means the written declaration of intention of any person to not receive any further medical treatment to prolong life in case that there is no hope or a cure or recovery. Such person wants to pass away peacefully. Such intention must be executed in writing when the person is conscious. It is legal under Thai law.
Mercy Killing, also technically called in Euthanasia, is the case that where a person does not want to live any longer because of serious health conditions, and allows a medical professional person to end his life. It is not legal in Thailand. Nevertheless, it is legal in some countries, such as Switzerland.