A person who believes in or advocates for extinction (either of species, ideas, or other entities), or historically, someone who supported extinction of a species through active efforts.
From 'extinction' + '-ist' (one who practices or believes in). The term emerged in 19th-century debates about species preservation versus the idea of 'survival of the fittest.'
In the 1800s, some people actually called themselves 'extinctionists' when debating whether humans should try to prevent species extinction or let nature take its course—a surprisingly old controversy!
Complete word intelligence in one call. Free tier — 50 lookups/day.