Will astrocytoma kill me if I don't get treatment?

Get treatment. Astrocytomas are intrinsic tumors of the nervous system. There are several grades astrocytoma, ranging from benign, slow-growing tumors to more "malignant" behaving, fast-growing tumors. The grade of the tumor will determine its behavior and the treatment that is required. High grade tumors left untreated can cause death. Treatment is recommended for most astrocytomas.
Yes. These are cancers of glial cells in the brain. They can have different grades and thus be slow to very very fast growing. The worst is called glioblastoma multiforme. Depending on age, physical condition and grade of the cancer the first line is usually surgery. Subsequently a patient will need radiation and possibly chemotherapy. Unless its too advanced treatment is useful.