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.
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.
Hellboy has its dark moments, but it's not unanimously regarded as the absolute darkest. The perception of darkness varies from reader to reader, and other comics might push the boundaries further in that aspect.
A heroic sacrifice can be a good death for a story. For example, in many superhero stories, the hero might die while saving the world. This kind of death gives a sense of purpose and nobility to the character.
One of the top deaths in fiction is that of Sirius Black in the 'Harry Potter' series. His death was quite tragic as he was Harry's godfather and was killed by Bellatrix Lestrange. Another notable one is the death of Ned Stark in 'Game of Thrones'. His beheading was a shocker and set off a chain of events in the complex political drama of Westeros. And the death of Spock in 'Star Trek' is also very well - known. He sacrificed himself to save the Enterprise.