WordNet records no synonym and no antonym for bitter end. 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.
the final extremity (however unpleasant it may be)
“he was determined to fight to the bitter end”
(nautical) the inboard end of a line or cable especially the end that is wound around a bitt
Synonyms, antonyms and the broader and narrower relations for bitter end come from Princeton University’s WordNet. Full sources and licences.