[dylan604]

It's kind of like nuclear warfare here on Earth. If you want to eliminate every living thing, then sure do some sort of scorched Earth type of thing. However, that leaves the planet in an un-inhabitable condition.

If you need to wipe out the inhabitants but leave everything else so you can now use it, you need to not destroy everything in the first place. Otherwise, you now have to terraform a planet that you chose because you didn't need to terraform it.