It depends on your perspective. Berserk is definitely dark, with its violent and tragic storylines. But some might argue there are other mangas that could be equally or even darker based on personal preferences.
Not always. While the MCU draws inspiration from the comics, there are often changes and adaptations made to fit the cinematic universe and modern audiences.
They contribute in several ways. Firstly, they keep the fans engaged between movie releases. They also introduce new concepts and ideas that might later be incorporated into the movies. For instance, some new villains or plot twists can first be tested in the graphic novels. Also, they can provide different interpretations of the characters, adding more dimensions to the overall MCU franchise. Moreover, they can attract new fans who are more into reading graphic novels rather than just watching movies.
One of the darkest Marvel comics is 'The Punisher MAX'. It delves deep into the gritty and violent world of Frank Castle with very intense and disturbing storylines.
Well, for a manga to be the darkest, it often features a lot of hopelessness, a cruel and unforgiving world, and characters facing constant and overwhelming hardships. Also, the art style might contribute, with heavy use of shadows and dark colors.