Sadism involves enjoying causing pain, reflecting a disturbing mindset, often disapproved of in society. The villain's sadism was clear when he smiled at the hero's pain.
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Not Physical Only
Sadism can also refer to inflicting emotional distress, not just physical harm. His relentless insults displayed a form of verbal sadism.
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Metaphoric Use
Sadism is sometimes used metaphorically for harsh but necessary actions not actually enjoyed. The teacher felt like a sadist as she assigned a lot of homework over the holiday.