How would you describe something that is deadly or extremely harmful?

The term "malignant" is used in medical terminology to signify something that is not only harmful but often poses a significant threat to health, indicating a serious or lethal condition. In the context of diseases, "malignant" is commonly associated with cancerous tumors that invade surrounding tissues and can lead to severe health complications or death. This makes it the most appropriate choice for describing something that is deadly or extremely harmful.

The other terms, while they may have their own meanings, do not convey the same level of danger. "Benign" refers to conditions or growths that are not harmful or cancerous. "Malicious" typically describes an intent to cause harm but does not specifically denote a health-related context or the severity of the harm. "Non-threatening," as the term implies, indicates that something poses no danger, which is the opposite of what is being asked in this question.