General Antonyms:
* Harmful
* Malicious
* Malevolent
* Dangerous
* Hostile
* Threatening
* Noxious
* Virulent
* Deleterious
* Destructive
* Malignant
* Sinister
* Wicked
* Evil
Specific Antonyms Depending on Context:
* Malignant (for tumors or diseases)
* Aggressive (for behavior)
* Severe (for conditions)
* Harmful (for effects)
* Unfavorable (for outcomes)
The best antonym will depend on the specific context in which you are using the word "benign."