Cancer is basically u're own cells gone wild,
basically they lose the main regulatory principles of cell division, many factors can cause this, one of the factors are certain types of viruses.
Cancer is very hard to kill sine u need to kill u're own cells. Needless to say some innocent cells will die. Its hard even to halt its development, ye u can try to inhibit cell division, but many types of cells in the body divide fast - blood producing cells for instance.
Killing cells by inhibiting major metabolic pathways existent in all the cells in an organism is bad.
The only way I imagine we could cure cancer in the future is by nano particles targeted to the cancer cells. Some therapies are developed even now with monoclonal antibodies + enzymes that target to certain antigens on cancer cells, most promising is ADEPT method, but there is still a long way to go. And the main obstacle is cost of such treatment (every patient needs to get a treatment suitable for him).
And ye u can cure some cancers in the early stages (sometimes even in the late stages) if u somehow change how u're immune systems sees the cancer cells.