Why not? At the point the first guy left the first time, she looked to me like a normal girl who'd been abducted, chained up and left for a period without food or water. She seemed alive but weak. Maybe a bit deranged too, and maybe she didn't speak English. I didn't see any sign of power.
It was as if they'd found the victim of a serial killer. The first guy wanted to set her free and get her to a hospital, which made sense. The second guy wanted to keep her and rape her. The second guy was evil, basically. And stupid - she obviously hadn't chained herself up, so at least one other person knew she was there, and was liable to return. Since she was still alive, she probably hadn't been more than a few days without being watered by someone.
Then the second guy discovers she is a zombie - basically by beating her to death and noticing she doesn't die. I think it's clear he'd have done the same had she not been a zombie, and she'd have just died. His later actions confirm this. Which, OK, it's a horror film and it neeRAB evil and stupid people. But it did undermine the premise of the film somewhat for me. Her being a zombie became almost irrelevant, until the final scene (which wasn't enough pay-off for me).