WordNet records no synonym and no antonym for malpractice. 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.
professional wrongdoing that results in injury or damage
“the widow sued his surgeon for malpractice”
a wrongful act that the actor had no right to do; improper professional conduct
“he charged them with electoral malpractices”
Synonyms, antonyms and the broader and narrower relations for malpractice come from Princeton University’s WordNet. Full sources and licences.