Why did she turn in to bloody Mary?

Bloody Mary is a legendary girl (we don't even know if she was a real girl) who had a big obsession with herself and was constantly checking herself out in the mirror. When she died, her spirit remained in the bathroom mirrors, and it is said, When you stand in front of a bathroom mirror in the dark and say 'Bloody Mary' three times she will appear in front of you and she might or might not kill you. However, this is all just a rumour. Nobody knows if it is true.