Why was Mary Tudor Called Bloody Mary?

Mary Tudor was called Bloody Mary because she ordered hundreds of protestants and other dissenters executed during her reign. England had recently become a protestant nation under the rule of her father King Henry VIII, and upon her coronation, Mary was determined to return the country back to Catholicism. The change lasted until her premature death at the age of 42, when the country returned to Anglicanism under Elizabeth I, which is still the official religion several hundred years later.