Can a malignant tumor do serious damage to the body if waiting too long?

Yes: If a malignant tumor is not treated promptly, it can lead to a number of serious, irreversible consequences. For example, it might invade another body part (an important nerve, artery, or organ) and damage it. It might grow too big or spread somewhere else in the body, which might prevent surgery from being a treatment for the cancer. The sooner if is treated the better.

Absolutely yes: In many ways- obstruction to the adjacent organ from the increase size of the cancer; organ failure from cancer infiltration/replacement to the organ, decrease immune system and increase catabolism- cause weight lost and increase susceptibility to infections, erosion/invasion to blood vessel/organ cause bleeding, perforation, obstruction etc. Ultimately it can cause death.

Yes!: The definition of a malignant tumor is that the cells lose the normal control over multiplying. This means that they can spread, or metastasize, elsewhere. If not treated, cancer can kill. In order to answer your question more precisely, we would need more information about what kind of malignant tumor you are talking about and where it is located. See a doctor now if you haven't already.