Because cells that grow out of control will eventually take over and destroy healthy cells that actually serve a function.

It's because cancer cells break off of the original cancer and then spread throughout the body. This causes growths in vital organs like the brain or liver or lungs etc and will cause them to fail as they grow and you die. Usually, doctors will check the lymph nodes closest to the original cancer as lymph nodes filter bacteria/infection and broken off cancer cells as they pass through them. If you have cancer in the lymph nodes closest to the original cancer site then you can tell that some cells have broken off and started to spread throughout the body.

Leukemia is a cancer of the body's blood forming tissue, it causes an overproduction of white blood cells.

It’s like someone who comes into your house and eats your food and breaks your stuff and hides your bills. At first it doesn’t have a huge impact but eventually your power is cut off because you haven’t paid your bills, you have no food to eat so you are starving, you can’t properly rest because all your furniture is broken… cancer starts by just consuming the resources in your body, and it becomes overwhelming. It spreads to other areas and does the same thing.

It is so dangerous because your immune system doesn’t recognise it because it’s starts as your own cells. So with the person in your house metaphor, it’s someone dressed up like a family member so you let them in, you don’t throw them out even when you know there is a problem because you assume it’s not them that’s causing the problem.

Advanced cancer treatments function by ‘tagging’ the cancer as an enemy so your immune system fights it. But many treatments just try to kill everything in the radius of the cancer, while trying to limit damage to the rest of your body. So again with the house metaphor - you know the problem is in the corner of one room so you basically try to destroy that area without hurting the rest of the house too much. But there is still damage to the structure of the house that has to be repaired afterwards. This is why cancer treatments are so rough, You basically are killing off your own cells too, just trying to kill the cancer faster so that you can have time to recover.

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Medical student here.

Cancer becomes lethal when it impedes or prevents a vital organ from carrying out it's function.

Sometimes, where an initial tumour is won't do this. For example, some forms of breast cancer won't be lethal where they are. The tumour may be painful, and take away nutrients from other tissues and so cause other problems, but it won't be lethal. However, it can spread. The lungs are a very common metastatic (location other than the original site) site for all sorts of cancers, and here, tumours can block off airways and prevent you getting oxygen.

Cancers could also begin in an initially vital organ, such as brain tumours. Here, the cells keep growing and taking up space and nutrients until they kill nearby neurons. This can lead to personality change, memory loss, and eventual brain death.

Tumours can also be delicate structures. Often, the cells become hypoxic, and release a chemical (vascular endothelial growth factor) which causes blood vessels to grow into the tumour. Some types of tumours are delicate, and can bleed really easily. This could happen in say the brain, the lungs or the GI tract, and you can bleed to death either spontaneously (rare) or when the tumour is having its removal attempted (less rare.)

Another way cancer can contribute to death is when someone already has other co-morbities, for example, COPD (chronic obstructive pulmonary disease). Lung cancer in patients who are already struggling to get the right amount of oxygen into their blood or who are retaining carbon dioxide can exacerbate this problem and cause respiratory failiure, leading to acidosis and death.

These I would say are the four main ways cancer causes death.