Robb died because of a vow he broke. Hardly Eddard level of honour. Robb had no way of seeing what was coming, unlike Eddard. The entirety of Westeros is stunned by it.
He died for many reasons. He stupidly executed Karstark for killing the boys because of honor, which led to Robb losing much of his forces. He seems to have forgotten the Lannisters didn't play by the rules which they killed his father.
But Catelyn releasing Jaime was the final straw. I really doubt Tywin would have gone along with the Red Wedding if Jaime was still a hostage, and killed in retaliation.