From the very first sentence of his story Gregor Samsa has been in a nightmarish scenario. From the surreal qualities of his work and home environments, to his transformation into a bug and the side effects that come with it, the world he lives in is filled with dream-logic. Then, in the final scenes even though Gregor has already died, the nightmare continues in the underlying menace of his parents behavior. Examining the final scene after his death it is not difficult to imagine that the nightmare remains his, despite his death. The tone stays the same as when the story was told from Gregor's point of view, despite his death and the events that follow would certainly be nightmarish for Gregor.
Gregor's personality, and his sense of self worth and purpose, are completely defined by his role as the 'bread winner' for his family. However, in the final scene his family moves on without him, completely self-sufficient, even making a dig at his ability to provide for him by judging his apartment choice, portraying him as having held them back in their eyes. This is a nightmare for Gregor in itself. His family's lack of sorrow, and even relief at his passing would have caused no small amount of distress for Gregor, especially since it meant that he really wasn't an important member of the family to his family members.
His parents thoughts toward his sister are also consistent with Gregor's point of view, and a nightmare for Gregor. The parents only see his sister as a vessel for economic gains through marriage, and are almost parasitical in their observations about her. They seek to take away the future in something meaningful and stimulating like music that Gregor had sought to provide for her, replacing it with a marriage to presumably someone who she would not love, but who would be wealthy and important in someway. This scenario would also be nightmarish for Gregor, as he loved his sister very much, and it also aligns itself with Gregor's view of his parents as people who use others for their own gain, as he observed them doing to him.
These aspects of the final scene combined with the fact that the tone remains the same throughout, suggest that instead of ending the nightmare and having a happy ending for the family (as the surface of the final scene would have you believe), Gregor's nightmare adapts. Gregor's nightmare becomes more like one of those dreams where he isn't actually present in the situation and has no power to affect it, but is still able to observe it. Sort of like in movies where the main character has a flashback and it seems like they are there in the moment, but they aren't actually a part of anything that's happening around them, but in this case Gregor isn't self aware enough to realize that he's there, only enough to experience the ominous feeling that he gets from his parents' behavior.
