The answer to the specific question, "where should we list different reasons given for death" is simple: In the biography. With a source.
Categories group. The data field summarizes. The biography documents.
So your first question in addressing a profile, should be, "is this relevant information about a person?" If yes, it should be in the biography. If no, leave it out entirely. Then the second question should be, "what are the groups of people into which this person usefully fits? All categories are duplicative of what is in the biography, so the second question is about grouping, not documentation.
Now, if you think your profile would be improved by listing a cause of death, you have good news and bad news. The good news is there is already a category:
https://www.wikitree.com/wiki/Category:Causes_of_Death. The bad news is that there is little logic to the subcategories already listed, and therefore the chance that your profile would be grouped with similar people is diminished. There is a subcategory, for instance, for Old Age. But you'll never see that on a modern death certificate, because when the person's body wore out, as all of ours will, something stopped, and that's the cause of death. So perhaps you'll create a subcategory for Heart Failure. But did your person die of Congestive Heart Failure or Cardiac Arrest? Cardiac failure? Cardiopulmonary Disease? I just googled "Cardiac" and got 78,600,000 hits.
So before creating the subcategory, you'll need to answer both, "what category of heart failure will best group my profile," and "will enough other people find it useful to usefully populate the subcategory?"
Am I attempting to discourage you? Yes.
But if you really think it would be useful, go ahead and create the category. At the top of the profile type Category: Heart Failure. (Or whatever you decide the category should be.) Save the file and then view it -- what you typed will show in red. Click on the red, and it will take you to the new category box. Under edit, type Category: Causes of Death. Unless you want to restrict your group to Americans, then you could type Category: Causes of Death, United States. And save. Your category is now created.