I'm #starkset , but Robb had 2 major fukk ups
1. Beheading Karstark
2. Not respecting the political value of marriage by choosing some random ass brawd instead of the Frey allegiance.
Neither of those encourage other houses to have your back
The Freys were promised a marriage to a lord of a major house, not a king. When the marriage pact was made Ned was still Lord of Winterfell and Warden of the North. Edmure being the Lord of Riverrun was a higher position than what was promised in the original marriage alliance. Having his daughter married to King Robb was not part of the deal.