Why is chronic leukemia considered malignant if it grows slowly and doesn't metastasize?

Cell are abnormal: Leukemia is a cancer of blood and bone marrow. These cells are not normal and are not found in normal blood and marrow. Usually there is some type of dna damage in these cells that causes the cancer. Eventually leukemia will destroy the bone marrow, making it impossible to make normal blood cells. That is the key feature, it's progressive, destructive and eventually fatal if not treated.

Cells don't die: There are 2 main characteristics of malignancy: 1) the tumor cells are genetically altered from normal, and 2) they don't know how to die and therefore accumulate, and impair the survival and function of their normal counterparts. All chronic leukemias fit these criteria.

See below...: Chronic leukemia is a form of cancer and therefore malignant. "metastasis" is a term applied only to solid tumors that occur in the form of a mass in a certainly location and then travel to other organs. Since leukemia is a form of blood/bone marrow cancer, it is already traveling in the blood in the form of separate cells (not a mass of cells) and technically it is already metastatic.