Too bad this is the non-spoiler thread because, to paraphrase Ygritte, You know nothing.
---------- Post added 2013-06-03 at 02:10 PM ----------
The Boltons have always had a rough relationship with The Kings in the North. A long time ago they used to war with them and their sigil, the Flayed Man comes from skinning any Stark they would come across. Granted they've lived at "peace" with the Starks for around 1000 years but Roose is an ambitious man and he saw this as a good time to take advantage and take back the North.