The correct antonym for BENIGN is Malicious.
The word Benign describes something that is gentle, kind, or harmless in effect. In a medical context, a benign tumor is one that is not cancerous and will not spread. In general usage, it refers to a mild, favorable, or non-threatening quality. For instance, a benign smile is kind, and a benign climate is pleasant and not harsh. It conveys a sense of goodwill and lack of harmful intent.
In stark contrast, Malicious describes something characterized by malice; intending or intended to do harm. A malicious act is performed with the deliberate intention of causing injury or distress to someone. This word embodies ill will, spite, and a desire to inflict suffering, directly opposing the gentle and harmless nature of 'benign.' While 'Harmless,' 'Kind,' and 'Gentle' are all synonyms or closely related to 'benign,' 'Malicious' stands as its clear and direct antonym, representing active ill intent.