To be fair, they didn't straight up intend to nuke the planet. They just wanted to disable what was going to be a huge space nazi fleet attack. The KZ franchise has always been weird in the sense that it tries to make the audience somewhat sympathetic towards the space nazis. But at the same time, they're space nazis who commit way more war crimes than the Vetkans. So fuck em. It's borderline tone deaf to go "you were the bad guys all along" when it's juxtaposed against this imagery:
^
This game goes harder than ever with the "actually the Helghast have some good points" bit but simultaneously starts out with us giving part of the planet to them, and then they execute anyone who hasn't moved out yet. The writers of Killzone did not have the chops to make this premise work. Especially given that the extensive background lore is entirely absent from the games.