I'm so sorry about your aunt. Losing a loved one is always very hard for many reasons, and grieving for the loss is a process that can take a very long time and may often manifest itself in unexpected ways.

To answer your difficult question: cancer is actually a group of more than 100 diseases that affect all kinds of cells in the body, but what is common is the abnormal and excessive growth of cancer cells, which invade other parts of the body. People usually die from cancer because malignant cancer cells have spread to parts of the body in such a way that prevents those parts of the body to function normally (imagine having cancerous lung tissue in your liver, for instance). People can also die from the treatment for the cancer they suffer from; radiative and chemotherapies can be very aggressive and weaken the immune system tremendously, allowing secondary diseases (for example, even things like the flu) to result in patient death.

The abnormal growth of cells which characterizes cancer usually occurs because something has changed in the genes that regulate normal cell growth and reproduction. There are two broad categories of genes whose mutations can result in cancerous cells: oncogenes and tumor suppressor genes. Oncogenes are involved in promoting cell growth, whereas tumor suppressor genes inhibit cell survival. Typically, many changes/mutations need to occur in these genes before a cell becomes cancerous. There are genetic and environmental components to this, and many factors that can affect whether a person develops cancer or not. Some examples of these are lifestyle choices, such as diet, exercise, smoking, excessive time in the sun without protection, or exposure to certain kinds of viruses (such as HPV). Some people may have a genetic predisposition (may be more or less likely) to have cancer given the same environment as other people. Age is another factor; because people are living longer, cancer rates have increased as compared with rates in past centuries. As you can see, there are a variety of factors involved in the development of cancer; it is very situation and person dependent.

I hope this has helped to at least scientifically answer your question. I think everyone has his or her own idea about why people die in general; I'm sure you have talked about this with many people in your life, so I will not touch on that here. However, I do hope your aunt had a very full life -- one that was replete with joy, love, and wonderful experiences. And I hope that you get to live such a life as well; life is too short and precious not to live with wonder and love in your heart.