For a society such as that of the Seven Kingdoms of Westeros with its very limited formal capacity for legal administration, the availability of trust and custom is even more important. But as we saw on last night's episode, sometimes the gains from a breach of trust can be temptingly large.

When considering this matter, I think it's worth saying that Robb Stark's actions are arguably even worse than Walder Frey's. Whether he broke his original promise out of a misguided sense of honor (as in the books) or simply true love (as in the show), he's actively undermining the key social and political institution of Westeros. Arranged marriages run afoul of modern sensibilities and strike us as cynical. Turning such a marriage aside for honor or love strikes as us idealistic if perhaps misguided. But that would be a mistake. Marriage contracts are the only means that the major houses of Westeros have for forming alliances and ending conflicts. Walder extracted a high price from the Starks in exchange for his assistance in crossing the river, but the Starks were asking a favor of considerable value. The coin that Walder demanded—marriage arrangements for his children—was absolutely the standard sort of exchange for major political favor. A world in which one house promises another house a marriage alliance and then turns its back on the promise on a whim is a world in which houses are going to be perpetually at war.

Robb Stark made MANY mistakes from attacking King's Landing without sufficient armies, to trusting Theon Greyjoy with Winterfell, to beheading a loyal member of his army against his advisors' wishes.

It's so interesting that it's his breaking of his vow to marry Walder Frey's daughter that did him in, but...

We don't know the back story. Frey might actually be using it as an excuse and instead the real reason for the Red Wedding is the Lannisters paid Frey and Bolton to assassinate the Starks.

Shocking though it may be, this is an important turning point in the plot with many ramifications:

1. The remaining Starks have every reason to fight dirty now. Arya Stark will have her revenge!

2. Walder Frey just made it so that NO guest in Westeros trusts ANY host. Expect more fighting.

3. Looks like Melisandre's magic / divine meddling is working. Expect more magic... and more meddling from the gods!