Awful lot of killing just because Robb Stark did not marry one of Lord Frey's daughters.
It wasn't just that, in the books Walder's oldest son (one of the only ones he actually liked) led the Frey forces that joined Robb and died battling the Lannisters for him. The one he intended to inherit the Twins (and one of the only diplomatic, intelligent Freys as well) was now dead.
On top of that, Robb was planning on leaving the Riverlands to go back into the North and wipe out the Ironman invasion meaning that the Twins would be a major target of the new Southern Alliance to get back to the North and there'd be no Northern defense from Robb.
Along with the disrespect of his Marriage Vow, all that combined is what set Frey off.
It was a perfect storm for Tywin Lannister. Robb broke his vow, which angered Walder Frey. He executed Rickard Karstark, which alienated one more powerful bannerman. He had lost his home, Winterfell, to the Ironborn, which gave Roose Bolton the idea that it was time for someone else to rule the North. Tywin was masterful at coordinating all these aggrieved subordinates and uniting them in common purpose. Now, the Freys are placated, the Karstarks are revenged, and Roose Bolton is the big dog in the North. Not to mention the War of the Five Kings is basically over now.
I think G.R.R. just wanted to shock his viwers/readers. There was so much more he could have done with this characters but just like with Ned, he threw the white flag.
I am guessing now, based on the movie episodes: with out the Wolfs the conflict is pretty much gone now, right? Who is going to fight Lannisters?
Stannis is still out there. The alliance with the Tyrells is shaky. Dany has dragons and an army. The Others (White Walkers) are coming, eventually. The Brotherhood Without Banners is still on the loose. And of course, not all the Stark bannermen pulled a Roose Bolton and switched sides. The North remembers...