WordNet records no synonym and no antonym for assassinate. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
murder; especially of socially prominent persons
“Anwar Sadat was assassinated because many people did not like his peace politics with Israel”
destroy or damage seriously, as of someone's reputation
“He assassinated his enemy's character”
Synonyms, antonyms and the broader and narrower relations for assassinate come from Princeton University’s WordNet. Full sources and licences.