While it ended up not being cancer, I can speak from personal experience that sometimes you can't just let something kill you, especially if you get a life threatening infection- because the doctors won't allow you to die.
I had several large tumors in my ovary and around lower GI tract that had leaked fluid into insides, was this close to getting sepsis according to the surgeon. The pain I went through for months with the tumors was excruciating, I couldn't go to the bathroom hardly at all and my partner had to give me enemas, which was incredibly painful humiliating and triggered my ptsd like crazy. The healthcare system constantly delayed everything and left me suffering for months and months.
When I was finally allowed to get an MRI scan, all of the medical staff were extremely worried I had cancer because of the appearence of one of my tumors. My red and white blood cells were out of wack as well and I was eat up with inflammation, so I had to go through a battery of blood tests to find out if I had ovarian cancer, and my tumors had to be biopsied. To be honest, I was begging for death. I didn't want to have surgery at all and would have easily let the tumors kill me since I have enough health problems already and would have been glad to finally be done, but it would have been agonising, slow, and painful death from sepsis.
Doctors won't really let you die or give you palliative care unless you have something imminently terminal that's incurable. I was pretty much told that if my condition got worse I wouldn't really have a say in what happened to me, lifesaving treatment would be forced because it's assumed that everyone wants to live by default and if you don't want something treated you're not of sane mind.
Everyone thought I was insane during the entire process. Even when doctors and techs were flitting around the prospect of cancer, I was at peace with my decision to not get treatment if that was the diagnosis, and I was not afraid to die. After all the pain and suffering I went through especially with the tumors, I just wanted to be dead and still do. Doctors and laypeople both did not comprehend this.