Sardonic as a sentence?

Rupert was being thoroughly unpleasant, and the sardonic smile never left his face.

Great satirists save their most sardonic wit for the greedy, the corrupt, and the hypocritical.

Some people think he's a brilliant comic, while others find his sardonic sense of humor to be unnecessarily cruel, and his overall bleak outlook on life more depressing than funny.

The man had a sardonic grin on his face. Professor Snape had a sardonic smirk on his face after Harry melted his cauldron.

When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).