She is entirely to blame for Andrei's death. If she hadn't been so stupid as to take up with the clearly knavish Anatol Kuragin, Andrei wouldn't have gone back into the army and gotten wounded. Again.
Plus, clearly if you nurse a man back to health and keep him from getting gangrene and stuff, and he just gives up anyway, even though he has forgiven you for your seriously deranged peccadillo, it is because he simply cannot face the thought of spending any more time with you, because you're awful.
Not caring one way or the other about Boris Drubetskoy--whatever. Cheating, if fakely, on Andrei Bolkonsky--probably you should just be put down for the greater good of humanity.