when you actually watch, and i'm watching all the episode 9's for some reason, Robb was killed for much more than just breaking that oath
they roll up to the frey castle and hope to cross the bridge.. everyone is warning robb about frey. Cat says "be careful.. we can't risk sir frey sending warnings to the lannisters"
robb makes a comment about frey being a bannerman and "shouldn't we expect his support." one dude even says "never expect anything from frey and you'll never be surprised" ohhhh the foreshadowing
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another guy warns of entering alone and he'll sell you to the lannisters... even theon warns him not to go in
the next scene.. you see who frey is... he was pissed off the second he saw cat "what is your son too afraid to come ask me himself" he was evil back then... talking shyt to his kids, to his wife.. HE WAS PISSED HE WAS NEVER MARRIED INTO THE TULLY'S.. he was pissed cat's dad never came to ANY of the weddings.. he said the tully's "have always pissed on him" then stated how mad he was that their families were never married
he then proceeds to shyt on the oath he swore to her father.. "yea i said some words... i even swore to the crown too, now joffery is king.. if i had sense i'd sell you both to the lannisters.. (he names all the houses) give me a SINGLE reason why i should waste a thought on any of you"
he was trying to tell them then.. fukk all these families.. what are you gonna do for me now.. cause i could easily get rich off you any day off those lannister boys... so when he finally was gonna be apart of the tully's and basically have his family, no longer bannerman, but actual family.. he took that over his other options
then he gives robb troops.. ALL of his troops except 400 to stay at the castle.. and he sends his son with robb to be his squire (spy).. arya was supposed to marry the frey's other son (another wedding that was fukked up) and robb was supposed to marry a daughter
then robb fukks that up. like he says at the wedding, she was supposed to marry a king, and now i have the dumbass uncle and no arya.. frey quickly just went back to tywin like, yea what you got to offer. he never gave two fukks about robb or anybody else. he told them from the jump, and they all knew from the jump, he was crazy and gave no fukks..
and still robb decides to cross this man. then roll back up to the crib like it was all good.. but when you rewatch you also notice, robb never met frey... and the whole time they was telling him, he took him as a complete joke... who's laughing now