I wouldn't consider the deliberate eradication of one's own species to be the equivalent of 'saving all life'. Especially when the entity responsible for the act of genocide was possessed of knowledge that could have been exploited in order to genuinely find a way to at least make a genuine attempt to save everybody.

There's humans in our world who are possessed by the same thinking patterns as Hermes and Venat yet that doesn't mean that it would be justified for humanity as a whole to be wiped out to make way for something different based on the unrealistic expectations of self-sabotaging and self-loathing saboteurs.

As an example, I don't believe that extremism is required in order to carve what they declare to be a better path forward. I'm sure there's many who, for example, might hypothetically support the destruction of oil and gas pipelines in the name of the 'greater good' only to proclaim that anyone dying from the realistic consequences of lack of access to heat and food in the aftermath is but an 'unfortunate but necessary sacrifice'.

Bringing it back to FFXIV, specifically, we're talking about the elimination of an entire species. Just as the Sundered weren't keen to roll over and die so too can the same be said of the Unsundered. Neither had any obligation to perish on behalf of the other, yet it is precisely because of Venat's actions that they were forced into a such a situation in the first place.