in what way was mary responsable for her own death?

I suppose you can say Mary got drunk and enticed Bigger but that's not good enough. Bigger begins to kiss Mary first, then he smothers her to escape detection by the blind aunt. You really can't blame Mary for this. Sure she crossed some social taboos but she certainly didn't cause her own death.

I would never say that any woman is responsible for her own death, unless of course she commits suicide.

Mary was the "white, upper class, modern woman." She ignored social mores and did as she pleased. You could say she tested them by flaunting previous societal taboos in the faces of others.

Mary scares Bigger. She's friendly, introduces him around, allows him to be her driver, invites him to dine with her and her boyfriend. She gets drunk with the men, makes out with her boyfriend in the car while Digger drives them around, and then arrives home to drunk to make it up the stairs. Bigger's mistake, is that rather than call someone to help him get her up the stairs; he carries her up himself.

But Mary is not the only one who is drunk, and Bigger, he's not only drunk; he's aroused as well. He's been out with a white woman, that didn't happen then. He's drunk, she's drunk, and he's been driving her around in a car full of sexual energy. The combination of all of the things leads him to kiss her..... her mother walks in (she's blind), he panics and he kills her to keep from being caught. That's what happened.

Was Mary responsible? Absolutely not. Was she responsible for being friendlier than was accepted? Yes. Was she responsible for being drunk and unable to take care of herself? Yes. Did she ask for the kiss or the death that soon followed. No, she did not. She is responsible for putting herself in a precarious position, but she does not have the responsibility of causing her own death. Bigger made that choice all by himself.