I musta missed the ep where this house hospitality was explained. Cuz all along I thought Robb & Catt were stupid for going back to Frey like shyt was sweet. I keep seeing it said in this thread that marriage pacts are broken all the time, but I dont recall hearing about any on the show being broken tho. Since Robb broke the marriage oath I figured Frey was justified in his revenge. Went about it grimey as fukk, but justified.
It wasn't just the hospitality. In Westeros when you go over another nikkas house and break bread and salt with him (like they showed in the 1st 10 mins or so of the epi), all of your guests are under your protection and law. It is one of the most sacred customs in the country.
In other words, the Frey's not only killed the Starks they violated a centuries old custom that was pretty much a de facto law...you don't harm your guests. Literally it had people shook to go to events at other castles in Westeros. Hell people across the narrow sea heard about it.