WordNet records no synonym and no antonym for human nature. It is the only member of its synset, 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 shared psychological attributes of humankind that are assumed to be shared by all human beings
“a great observer of human nature”
Synonyms, antonyms and the broader and narrower relations for human nature come from Princeton University’s WordNet. Full sources and licences.