WordNet records no synonym and no antonym for splat. 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.
give off the sound of a bullet flattening on impact
split open and flatten for cooking
“splat fish over an open fire”
flatten on impact
“The snowballs splatted on the trees”
Synonyms, antonyms and the broader and narrower relations for splat come from Princeton University’s WordNet. Full sources and licences.