Why did Mary (Bloody Mary) Kill so many protestants?

Well, she probably did it to ert her position (to show that just because she's a woman doesn't mean that she isn't powerful). There was large religious instability in that time, with one faith, going to the next. No, she didn't succeed in reestablishing Catholicism, in my opinion. She died at 42 during an influenza epidemic and Elizabeth the First succeeded her. Elizabeth made everything more or less equal to the Protestants and Catholics and there were many plots by Protestants to kill her. Look up the Babington plot. :)