Well, unfortunately for her (too bad, so sad. Not.) the death of someone during the commission of a crime can be charged as murder. It doesn't matter if it was an accidental death, if it occurred during the commission of a felony as in this case, it can be murder. This is the case in California, I don't know if this varies state by state.
She brought a weapon. Even if she didn't bring a weapon, if they scuffled and he tripped, fell, hit his head and died, it can be a murder charge because her intent was to rob him and she robbed him after he was dead.
There are many links I found, but here's one:
http://www.shouselaw.com/felony-murder.html
Felony Murder Rule
Murder During the Commission of a Felony
There are limitations, but it's an interesting rule/law in general. I think it's a good one.
My thought process is that if someone brings a weapon to commit a crime against another human being, then it doesn't matter if they planned to use it or not - they brought it, they know they have it and they will use it. If she had come unarmed then got in a scuffle found her way into a drawer the found the letter opener and used it...maaaaaaaybe she might have had a chance at a defense. Maaaaaaybe.