there is no singular reason why robb was doomed.
there were a few
-he gave slight to the gods by breaking his vows > freys turned sides
-he got betrayed by theon and lost the north and more importantly his heirs
-he did not deal with the karstark/jaime/catelyn matter appropriately
all these things together, he was losing the war because his men were dwindling, he could not strike at tywin because Edmure tried to be a hero, so bolton added up 1+1 and decided it was time to turn cloaks. he wouldnt have if the North was winning. The freys are just crazy but I doubt he would have killed Robb if he didnt realise that the boy was lost.
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