I was also struck by the dark tone of Daredevil, and I think there are two main reasons, aside from dark (and antiheroes) simply being "in" right now. Firstly, one of the showrunners is Stephen S. DeKnight. He's most famous for showrunning the Spartacus shows, and so it's clear his aesthetic goes towards the ultraviolent end of the spectrum.
Secondly, they're basing the series very much more on the Frank Miller run of the comic (e.g., Stick, the Hand, and Elektra were all introduced then)--even down to recreating some panels as frames. And unlike the original Silver Age runs of the book a few decades earlier, Miller's run became particularly dark and gritty in tone. This was, after all, the beginning of Miller establishing his own voice and would eventually lead to books like The Dark Knight Returns and Sin City.