I didn't include this in my original post (which would've made more sense) because it's been a while since I've played the game but doesn't one of your allies die as a result of saving Mantis? I could definitely see how it could test one's morals if they were playing a heroic route. Although, I do agree with you, destroying the hive seems to be the best option for a good playthrough.It really really doesn't test your morals at all.
The hive is an existential threat to humanity straight out of a horror story.
It makes the zombies look tame in comparison.
Exterminating it is more important than literally anything else you could ever do.
From both selfish and altruistic perspective, the hive must be destroyed.