In a manner lacking compassion, feeling, or enthusiasm; without emotion or courage.
'Deadhearted' + '-ly' (adverb-forming suffix), following standard English patterns for converting adjectives to adverbs of manner.
Deadheartedly describes the method of cruelty—not passionate anger, but cold, indifferent harm done as if it means nothing. It's the adverb of people who hurt others while barely noticing.
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