For something that's very harmful, especially a tumor that's cancerous, use the term malignant.
Malignant and its opposite benign are medical terms used to describe a tumor or growth as either cancerous or not respectively. The gn part of both words comes from the Latin word for born, differing in the prefix mal "evil" and bene "kind." A malignant tumor grows uncontrollably and spreads to other parts of the body. Though less common, malignant can also be used to mean "evil, malicious" like when someone has a malignant imagination.