In the late 21st century, Earth is at peace, sending fleets of Valerie spacecraft to the stars. Without warning, relativistic kill vehicle obliterate the surface of every planet, and a mop-up fleet of ships begins hunting down the humans that remain.
The last two humans on Earth are kept as zoo specimens. They ask why their planet, and all its species, were exterminated. The squid-like aliens explain their logic thusly:
1. Every space-faring civilization is capable of planetary extermination.
2. If given the choice, every species will choose to exterminate another species rather than their own.
3. Each side knows that the other has the same choice.
It was for racial purity. At the time eugenics was a big deal, it stated that some genes which were exhibited as external traits were superior. Hitler was attempting to create a better world by performing artificial selection.
As much as his ideas and methods were bonkers and would never work the fact that humans have stopped evolving naturally is something that some people still want to change.
People are still evolving. Evolution is adaption to existing conditions and is basically impossible to stop. In fact dispute billions of years of multicellular evolution the cells 'in your body' and given time will develop into cancer, then portable cancer inside your body, and given the right conditions portable cancer outside your body.
Why wouldn't it be a goal? If you're fighting over space resources and destroy their space-based military, then its safe to assume the planet bound will soon be launching missiles at you.
The obvious plan is glass the planet, spend 50 years harvesting the orbital resources and then you'll have the infrastructure in place to strip mine the lifeless planet.
