The inhabitants of the planet Braxon were well respected by their galactic neighbors for their highly advanced civilization and culture, as well as their technological achievements. Not only had they achieved world peace and found cures for most of the diseases that once ailed them, but they also managed to do a bit of space traveling, even installing an impressive system of planetary defenses that could destroy or divert any asteroid that might threaten them. The quality of their architecture was considered second to none, and to top everything else, they developed a commercially-produced tomato that tasted like something other than cardboard.
However, success sometimes brings its own problems; and the Braxonians soon realized that because of the absence of wars, pestilence and famine their species was consuming more and more of the planet’s natural resources to the detriment of the native plants and animals. Since their highly advanced culture recognized that all species were created equal, that there were more non-sentient species than sentient species on the planet, and that the needs of the many must outweigh the needs of the few, they concluded that only one solution would restore the balance of nature and protect the planet from all future encroachments: self-genocide.
So they spent the next twenty years winding down the business of their species, dismantling cities, sealing up waste disposal facilities to the best of their ability, and generally tidying up. Then on the Big Day the remaining population distributed and duly consumed the fruit-flavored drink with the unhealthy ingredient.
The next fifty years saw the planet attempting to reach a new environmental equilibrium, as once-domesticated animals tried to re-adapt to the wild, invasive species slowly reached the limits of where they could grow, and the suddenly enormous population of scavengers returned to more appropriate levels. The planet was well on its way to becoming a new natural paradise (except for the tomatoes, in which cardboard-flavor became the dominant trait) when an enormous meteor fell out of the sky and obliterated all life on the planet.