Eliminating the Guards and Awakened in the barracks wasn't that difficult. No one had reached the 3rd Tier, and none of the Awakened had an overly powerful Soultrait to counter Tiara and Michael's prowess either. They were overwhelmed with brute force, unable to do anything.
Their only fortune was that their deaths came fast. The Kitsun didn't have to suffer too long before they died. Michael and Tiara didn't have enough time to reconsider letting the Kitsun suffer and draw out their deaths. They didn't know how much time they were left with before the reinforcement of the Kitsun Lord would arrive.