That Karstark is pissed that his father was beheaded by Robb for killing Lannisters. This was all telegraphed way back when, when Robb was warned against his planned execution. He only wants the Starks to fall and the North to change (my opinion). I think it's fitting he didn't care that Roose Bolton was killed by his own bastard - I mean, the North had no love for the Boltons to begin with.
The Boltons were very loyal to Roose, true, but in the context of the show they have no idea Ramsay murdered him. But the entirety of House Bolton is rather ruthless, with their flaying and gleeful betrayal. They've also long been a thorn in the side of the Starks.
I dunno. I think it all works out in the show. If things go down even remotely like they do in the books I'm sure GRRM will flesh it all out a lot more. Like 50 pages worth of story more.
I'm well aware why the Karstarks are pissed, the guy even mentions that outright during their conversation, and it's shown in the preview. I'm not daft. But revenge against the Starks or not - would you really want that guy lead the charge? Would you really want that guy to be your liege lord, the man you answer to, especially considering that right now the Boltons are spent and overextended, and Winterfell is still in reparations, thus very easy to take? And to top it off, the Boltons have made themselves an enemy of the Iron Throne by marrying Sansa to Ramsay. And as icing on the cake, Ramsay wants to attack the specifically neutral Night's Watch, based on nothing more but an assumption, which should be an insane outrage to the Karstarks, even if a Stark bastard is Lord Commander.
My point is, ruthless and treacherous or not, you can't just go around and murder anyone you like if you are to be the leader of a noble house, especially one in a massively powerful position such as Warden of the North. Or rather if you do, make damn sure nobody fucking sees it.
Joffrey was ruthless too, and even he was a lot smarter about his blatant attempts at the lives of people he didn't like than Ramsay. The show makes Ramsay appear to be a blathering, drooling fool who has absolutely no understanding of how power works.
As with the Martell murders, there should be a massive political fallout - but unlike a civil war breaking out in the North, this right here should lead to a completely united North, standing together in order to take back Winterfell and install either Sansa as a puppet, or just find a new Warden of the North.
In other words, I'm just sick of nobody having any direct consequences for the blatant disregard of political dynamics by just murdering liege lords and family members. Everyone there sees them, but no one gives a single fuck. I know that the world of GoT is brutal and unforgiving, but there's being brutal and unforgiving, and being just downright daft and being brutal for the sake of brutality. Pragmatism only goes so far, even in this universe. I would think Joffrey amply displayed that.
I know Ramsay will get a reckoning, but I believe that this reckoning will not happen by him getting betrayed by his own people and allies. It will happen by a relief force made of Jon and the Wildlings and probably the soldiers of the Vale.