Did you hire her thru an agent? I would tell your agent that she has been acting really strange with the bruises and make it appear that you are keeping her but am worried about her. Hopefully, they will have some clever way to sort it out, cos they must have seen so many cases like this.
I guess you can also speed it up on your terms, call the police and tell them she has been acting strange with self-inflicted bruises and you are going to fire her and you fear she may do something dangerous, then when they arrive, fire her in their presence. That way,if she is planning no good, she would not have the time to prepare, if she is not, then it's ok too.
Not very clever ideas, I am afraid.