The real genius in this is that it only IMPLIES; it does not TELL. We can guess what the ending is - the last panel makes it pretty clear what the ending is - but it doesn't actually tell us. It leaves the ending to ones imagination. We know what it SHOULD be, but it COULD be something else. Maybe Marion ISN'T dead and dismembered. Maybe she's just hiding behind the furnace or something. Maybe, maybe, maybe - it's all in the maybe. Without that ambiguity, the story wouldn't be nearly as effective, because if Marion ISN'T all cut up into pieces, then what they're all looking at is EVEN WORSE - and your imagination can fill in just what that might be.