Apollo I managed to read the post before.

Destroying all members of a hostile species(assuming it was non-sentient) would be disastrous. You could be wiping out a potential predator of a smaller species, leaving it to breed without having any predators and spread rapidly. This sucks up the natural resources and causes problems for all other species. And you would not just be wiping out one predator, you be wiping out quite a few different species. You could also end up considering territorial species as aggressive, thus leading to you killing them.

Basically, your completely fucking the eco-system of the entire world before you have even colonized it. If this is really what the Septonians have done that each and every one of their colonies and planets will be going through terrible ecological disasters.