“The Engineer and the Executioner” by Brian Stableford

First appeared in Amazing. Reprinted in genre retrospectives by Donald A. Wollheim, Mike Ashley, and Josh Pachter. From the collection, Sexual Chemistry and Other Tales of the Biotech Revolution.

Summary:

Here's a story that improves the more one ponders it.

A robot has sentenced Gabriel Samarra to deportation and his life's work to death as they endanger earth. On the hollow asteroid Lamark, Gabriel has created an ecosystem that evolves new species generation to generation. The plants take over and absorb their environment; therefore, should these creatures happen to fall upon Earth, its creatures would soon fall prey like rabid invasive species. The robot will allow Gabriel a last moment to gather his things in order...

Commentary with Spoilers:

As you might have guessed, the prisoner kills his captor (although escaping seems another likely scenario). A ricocheting bullet grazes his own leg and dooms himself. He breaks the glass sealing his creatures inside crawls out to join them. While the robot's ship still tips the asteroid into the sun, plants take over the engineer's compartment and eventually eats its way to the outside, creating a panspermia event.

Although the inital scenario feels dated, the evolution and the structure intimated in the title should make the story worth reading. Gabriel is the presumed good-guy engineer, preserving life, and the robot the baddie executioner. But by the story's end, the roles and the reader's sympathies actually reverse.