Matthew Welch/AMC
In the comics, Castle - who has been previously portrayed by both Tom Jane and Ray Stevenson in "The Punisher" and "Punisher: War Zone," respectively - is a special forces vet who loses his family to violence brought about by organized crime. Believing the law doesn't significantly punish criminals, he begins a one-man war on crime, executing his targets so they can never hurt anyone again.
Marvel
This means we'll likely see a brutal showdown between the two characters, since Daredevil, while operating outside of the law, very much believes in it, and is certainly against killing. We don't know much about the plot of "Daredevil" season two, but if there's a Big Bad involved, then this also sets the stage for one of the greatest of action-movie tropes: The uneasy team-up.
Pretty exciting stuff.
Season 2 of "Daredevil" will arrive on Netflix in 2016.