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.