The serum still works if (1) Homon hasn’t counteracted it and (2) there’s only one berserker spirit possessing the body at a time. These are important qualifiers.

In retrospect, it’s interesting that Rabbit and the other adventurers don’t know as much as they don’t know. Byron would’ve had to remain closeted to the adventuring community at large, and his fellow Peacemakers would’ve had to aid in that little lie-by-omission for the sake of the Guild. Even Bandit understands that “our leader is a few missed meds and a bad mood away from killing us all” might not be the best bulletin to share for the sake of morale.

But the mask slips a bit in the last panel. Originally, Bandit’s line was “Neither had I,” underscoring that she had told no one about her death by Byron’s hand who didn’t already know about it, probably not even the newer Peacemakers she recruited. She vented about the experience to Byron privately and in front of Gravedust, but that’s about as close as she came to unpacking it, as far as we know.