Tumor malignant degeneration means what? Is the tumor dying on its own?

Cell death. Malignant tumors have various types of death. Comedonecrosis is a cell death in the middle of the tumor (due to outgrowing its blood supply). Dengeration just means it has died, but in general due to senescence (just old age). Tumor necrosis is usually something seen in malignant tumors, while degeneration is usually associated with benign tumors.