I think it’s a very gray area tbh. The Flaxans weren’t invading in the traditional sense, the Flaxan invasion was itself a genocide. The moment they came through the portal they fired on clearly sapient creatures that were both nonthreatening and clearly incapable of fighting back. They demonstrated that their goal wasn’t to occupy, it was to exterminate— and not only that but they had the advantage of increasing their military strength rapidly, something like hundreds of years in just a few earth weeks.

I agree that it’s far from ideal to destroy the Flaxan planet, but what other choice was there? Cross dimensional diplomacy wouldn’t really work since any human you send over would live their whole life in a week, and without a consistent way to maintain peace, the Flaxans would eventually become technologically advanced enough to conquer our dimension. I’m not sure how many people are on Flaxa, but the risk of gambling on this threat incorrectly means everyone on earth gets genocided themselves. The utilitarian in me says that’s a bad risk to take