Why was Henry VIII's daughter known as Bloody Mary?

yes

2nd Answer:

Yes, after she was Queen, she decided to change the country's religion from Protestant to Roman Catholic. The protestants were outraged and started protesting and petitioning Parliament.

So Mary ended that little rebellion by killing 100 protestant leaders.

BUT, most of Mary's bloody deeds were not even done by her, but rather by a certain Countess Elizabeth Bathroy. Bathroy is said to have bathed now and then in the blood of virgin girls, among other horrible things.