Cancer cells, unlike other cells in human beings, are anaerobic. That means they can not use oxygen when they metabolize glucose (burn sugar) for energy. A healthy, oxygen burning cell can produce 38 ATPs (energy packets) from one molecule of glucose and one molecule of oxygen. A cancer cell can only produce 2 ATPs from one molecule of glucose in the absence of oxygen.
Obviously then, cancer cells are very inefficient at producing energy. As a consequence, they need 19 times more sugar (glucose) than non-cancerous cells.
Cancer cells are both defective and primitive. Not only are they unable to use oxygen to metabolize glucose into energy, but they are also either lacking or completely deficient in many enzymes that normal cells have in abundance. One such enzyme is catalase which transforms oxygen and water into hydrogen peroxide and vice versa.
High doses of ascorbic acid (Vitamin C) result in the production of peroxides in and around cells. Healthy cells use the hydrogen peroxide for certain metabolic needs and the excess is converted into water and oxygen (good stuff).
Since cancer cells have very little catalase, they are unable to convert the hydrogen peroxide into water and oxygen and consequently are oxidized and killed.