Hmm....clear it may not be, but my interpretation has always been that she may have felt guilt over poisoning regan, and so killed herself.
But then, I may be completely wrong - perhaps she realsied that Edmond was not truly in love with her, and she was with him - perhaps she suicided from a broken heart? (doesn't really sound LIKELY though...Shakespeare doesn't portray her as a very loving charatcer...)
Oh, maybe...nope. Can't really come up with anything else..
I think I have now successfully confused myself....Hope it helps (in a roundabout sorta way...)
