Movies
When I buy a new book, I read the last page first. That way, in case I die before I finish, I know how it ends. That, my friend, is a dark side.
— Harry Burns ‧ When Harry Met Sally...

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Reading the ending first turns a small reading habit into a joke about death. The speaker wants the story settled in case they don't live to finish it. Calling this a dark side adds self-mockery to a practical solution for an unusually gloomy concern.
Context reviewed