When Walter first got cancer at the beginning of Breaking Bad, we see medical evidence, and also have no reason to question this being true. Same when the cancer goes away: Walter’s anger — caused by the cancer gone — is understandable given his new situation.
Towards the end of the show, however, we learn about cancer relapse only via Walter, who has in the meantime become a character who is not afraid of lying, or much worse.
Is there any evidence of this relapse? Because if there isn’t then in fact the viewer is presented with two very different interpretations of the show, and while Interpretation 2 (below) is way more powerful, it is basically up to us to decide which one we believe, which i find truly fascinating:
Interpretation 1: The cancer relapses. All the way from the beginning, Walter knew he had nothing to lose, and this gave him the motivation to have no fear, and while first the cancer went away to extend his adventures, the show, and add a curious twist; after all the same end finds him that he expected from the beginning, and the gunshot is just a graceful way of dying instead of waiting for the cancer to finish the work.
Interpretation 2: The cancer is gone for good and Walter lies that it relapsed. This is a transformation story beyond a point of no return: Walter first thinks that the cancer gives him nothing to lose, triggering his transformation to a criminal. However, even though the cancer goes away, he is now unable and/or unwilling to return to normal life, getting excited about “the empire business”. When he realizes he has no chance to survive freely long, he lies to his family about the relapse, so he does not have to face their blame that it is entirely his responsibility that he dies early.