Stein kinda just went out there, stole a body, stole a brain, and haphazardly threw them together without any research being put into whose brain he was putting there. Frankie could have been anyone, from a five year old, to a forty-eight year old serial killer. He didn't know. Despite his often terrifying intelligence... he's really just a kid. A bit of a moron too, if we're being honest. And he possibly just ruined someone's death, by putting them in a body that isn't even their own.