system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a deadly cancer like melanoma actually kill you?

What's going on in your body that actually causes your death? Why are some tumors benign and others malignant? What about a tumor makes it malignant?
gpt
A malignant tumor is one that spreads to other tissues.  This can either happen directly to nearby areas, or it can happen by metastasis. Metastasis is when distant tissues can be infected by, for example, crossing into the blood stream via the lymph nodes.  There they can spread to dangerous area like the liver or the brain.  Once they get to these new locations they continue to grow, crowding out the normal cells that should be growing there, and causing organs to fail.