What killed Mrs. Stolz?

Did I miss something? I we to assume she had a heart attack or suffocated? I still find those a little tough to swallow. Also, why did they setup Arthur again and shoot him? He wasn't paying rent? I admit I did not really pay much attention throughout the film on my multiple viewings. For some reason, its my least favorite of his and yet strangely its style is my favorite of his. I wish he had pursued the premise with more of the artistic integrity he used in say, Alphaville (a bit more toned down perhaps).