I suppose it was because America chose to help the hunters rather than save Akane once he knew I was one, but...I'd actually come up with that plan the minute I found out Kirigiri had been our former vigilante and was also armed. We'd originally planned to use it only as a last resort, but since the bloodhound had likely already identified both Ukyo and I, we chose to sacrifice her in order to convince the rest of the town that Kirigiri had never been converted. Without the bloodhound, it should be impossible to prove otherwise now.