i'd say that the sith do have ways of cheating death and the jedi would deem these ways unnatural. palpatine pretty much does what he says he can do, in dragging vader back from death by turning him into a cyborg whose parts can be replaced well beyond the normal lifespan of a fully organic person. the frankenstein homage at the end of ROTS isn't just a lark. what is frankenstein's monster but a grotesque imitation of life with a twisted fragment of a soul, which goes completely against nature, not unlike vader?