You're asking a good question. When a cell dies, it's because it loses the ability to make ATP, the product of oxidative metabolism and glycolysis and is sometimes called the "universal currency" for cellular reactions. The lack of ATP results in many things going wrong, including the cell's ability to maintain the integrity of its membranes, but many other reactions that are deleterious to the cell also take place. The cells that are most vulnerable to this are those that are the most metabolically active, which include the brain and heart. When these fail, many of the body's systems also fail and death may be declared when an individual has lost circulatory, respiratory, or CNS (brain) function.