Johnny noticed Kazuya's attention on him which made him remember the arm still near. That was probably why. Johnny could fuse his limbs back onto his body with nothing but a touch because Belial had given him that ability. If the same demon was what was powering Kaz now, it stood to reason that would make him want his arm back so he could do the same.

As much as he worried for Deso and wanted Otoha not to kill the teen, he took advantage of their distractions to retrieve the arm. Once it was close, the sharp smell of demon's blood told him it hadn't all drained out of the arm just yet.

...It would be a good idea...as disgusting and tainted as demon's blood made him feel, it was an energy shot and restorative he couldn't afford to pass up.

Johnny's fangs elongated and he dug them into the flesh of the arm to drink as much from it as he could. Of course, he hadn't anticipated the differences between the versions of demon's, so the scalding heat nearly made him pull away, but he forced each burning gulp down till he'd drunk what was left. Already, he could feel the skin patching a little faster and heat spread through him as the hot blood moved into his system.

He was going to hurt like a bitch later, but at least he could be more useful now.