mercy has 2 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
leniency and compassion shown toward offenders by a person or agency charged with administering justice
“he threw himself on the mercy of the court”
Synonyms, antonyms and the broader and narrower relations for mercy come from Princeton University’s WordNet. Full sources and licences.
a disposition to be kind and forgiving
“in those days a wife had to depend on the mercifulness of her husband”
the feeling that motivates compassion
something for which to be thankful
“it was a mercy we got out alive”
alleviation of distress; showing great kindness toward the distressed
“distributing food and clothing to the flood victims was an act of mercy”