What happens if you just ignore a skin cancer mole?

Don't take a chance! If a mole or any lesion on the skin has been biopsied as a cancer you need to follow through with the recommended treatment. This usually is complete removal by surgical excision. Melanoma kills so have any concerning lesion checked by a dermatologist and if the biopsy comes back as an early melanoma have it excised as this is curative. Don't take a chance.
Spreading. The worst scenario would be the cancer spreading to distant sites/vital organs, and causing death. That would happen to 25% of melanoma patients. For persons had other kind of cancers like basal cell carcinoma or squamous cell carcinoma, they might have a lesser chance of death, say 1/1000 or less. But bcc and scc, if not treated, can cause significant local tissue/skin disfiguring destruction.
Hard to know. Your phrasing makes the ? Difficult. Most likely you refer to melanomas, which often look like a dark mole. Technically, a mole has no cancer, and so won't ever spread. Melanomas will eventually spread and become life threatening, although how quickly that happens is varies widely. Some spread early by growing deep quickly, others may grow much more slowly. Best wishes.
Progress. It will progress and possibly travel to distant locations in your body and create new tumors.