assassin has 2 distinct synonyms spread across 1 of its 2 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.
a murderer (especially one who kills a prominent political figure) who kills by a surprise attack and often is hired to do the deed
“his assassins were hunted down like animals”
a member of a secret order of Muslims (founded in the 12th century) who terrorized and killed Christian Crusaders
Synonyms, antonyms and the broader and narrower relations for assassin come from Princeton University’s WordNet. Full sources and licences.