Who is more noble: Squealer or Yang Wen-li? (spoilers for this entire series and for series Legend of the Galactic Heroes)

Squealer did not have the luxury of avoiding casualties, since the imbalance of power between his own side and his enemy's side was of a greater magnitude than even the imbalance of power between the Iserlohn Republic and the New Galactic Empire. The only way Squealer could ensure freedom for his (sub-)species was to eliminate every single cantus user that lived. Leaving even one alive would mean risking defeat, since cantus users are so incredibly powerful that even one had a good chance of being able to kill all of the monster rats in retaliation.

Yang's risk of losing was never going to be complete annihilation of his personnel and countryfolk, so he would never have a motivation to do the same to his opponent. Yang also didn't have the total annihilation of his opponent as his only viable victory condition - he only needed to kill one man, really, to win his war. Perhaps Yang would have attempted xenocide, like Squealer, had that been his only means of preserving democratic republicanism (or the human race).