Cells have a variety of processes going on inside of them to do things like maintain homeostasis, replicate, fix DNA errors, produce power, etc. They even have machinery to self terminate when something goes wrong.

In a cancer cell one of the processes has broken down and the self termination process has ALSO broken down. Once that's happened you essentially have a rogue cellular factory inside of your body that can hijack the resources you need to survive and can replicate itself going from one cell to enough to kill you by any number of factors. The cancer could just physically put pressure on organs like the brain. It could produce a wild amount of signaling hormones therefore causing secondary non-cancerous cells to follow erroneous instructions. It could also take over resources that other parts of your body need to survive.