You are a mortal because you are mortal. This means that you are an individual destined to die because you are susceptible to death.
The word mortal has one of the most ancient genealogies of any word in English or any other language. It is related to English words like "murder" and "mortuary," and to the French, Italian, Spanish, and Russian words for "death." Something that can kill you can also be described as mortal. If you're in mortal danger, you're in a deadly situation. Action heroes often think they are dealing their enemy a mortal blow, but sometimes the enemy manages to come back to life. People are called mortals because at some point, they die.