How did Captain Marvel die?

If you're talking about the Captain Marvel who died in the 'Death Of Captain Marvel' graphic novel(there are a few others, even within the Marvel Comics company itself), then the man who Captain Marvel acquired cancer from was called Nitro.

Nitro had the ability to cause explosions(or something like that), and Captain Marvel developed cancer from his repeated exposure to him through their constant battles through the years.

However, this may have led to the circumstances which caused his death, but I wouldn't outright say that "Nitro killed Captain Marvel" in such a deliberate manner...realistically, Nitro didn't up and *decide* to "give" Captain Marvel cancer, it was just a by-product of things that happened.