You must be a show watcher and not a book reader. :tv:
There's a reason why the Red Wedding occurred. Tywin Lannister knew he could not defeat Robb Stark in battle (Yes, the mighty Tywin Lannister of Casterly Rock) and so he knew that the only way to defeat Robb was to capitalize on his mistakes. Lord Walder Frey has always been a man of questionable loyalty. At the trident he did not appear until the battle was over and Robert had killed Rhaegar. He's a bitter and a prickly man who dishonoured himself by breaking the sacred guest right and killing his guests who he swore to protect under his halls. No god, whether old or new, or even the Lord of Light could forgive him for this, as this is an act which is severely disliked and disapproved by the gods of the Westeros. There's a reason why we heard 'Robb Stark is undefeated in battle' for the past three seasons. No army could beat him. His tactical senses far outweighed those of the South. In the end, it was treason and betrayal that lost Robb Stark his head. The only reason Lord Walder decided to slaughter the Starks was because he was angry at Robb for breaking his wedding oath. No reason shall ever justify the Freys betrayal. The north shall remember. To sum this up, both Tywin Lannister and Walder Frey are dishonourable men. The latter is also cowardly and of questionable loyalty. He is a prickly man and his line shall end, for the night is dark and full of terrors.