In one of those TMI, gotta pee and will be running to the bathroom in a bit but... dunno about needing a living body but I'm estimating the cellular/metamorphosis layer mimics tissue, and unless that person wants to mimic dead tissue, they'd need a living body. I don't think a shapeshifter needs to keep the 'template' alive afterwards as evident by Nightshifter's 'body in the ceiling'. I think they may keep a person alive as a 'scapegoat' or they just aren't the serial killer type.