If a suspected mole is left without being removed, and it turned out to be melanoma, what will happen to the body, what the person will feel?

Varied. In most cases you would feel nothing. Most melanomas are found early. There may be more surgery required depending on the situation.
Risky . If it proves to be a melanoma you may have missed the chance to cure it surgically. If it spreads into the body, you are not likely to feel much, but are very likely to die from the tumor.
Progression. The melanoma would grow deeper and then the melanoma cell would start to travel to other organs and create metastatic disease. Symptoms would depend on the location of the metastatic disease.
Your choice. You will feel nothing. However, there's a case to be made for removing any mole that the person is actually worried about, after discussion with a physician or two, whether or not there's any realistic concern that it's a melanoma. This not only ends the worries, but also helps increase awareness of this dangerous disease and the life-saving practice of excising suspicious lesions.