From what I've read, I believe that she was killed on her way to a school dance.
What I know about Anna is that she was killed tragically on her way to a school dance and she is trapped in her house and kills anyone who steps into her house. But what i've learned is that Anna does not like to kill people and that something in her forces her to kill all these people. Anna is a good person who died tragically and for no reason.
I learned that Anna is conflicted between being an evil killer and someone who is confused about life. Anna didn't want to kill Cas, so it shows that she stilll has somewhat of a heart. She just has a hard time of resisting her murderous impulses.
A random psychopath attacked her and slit her throat on her way to a school dance. I think this because those are the most clues I have read about so far.
So far, I've learned that Anna has two different sides to her, almost like a split personality. The one Cas talks to and the one that murders. She had many dreams when she was alive and turns into her 'ghost form' and tries to kill anyone other than Cas that enters the house.