First of all: He put his own interests ahead of the wellbeing of the men who followed him, making him a very bad leader. He knowingly risked to throw away a military alliance which would have been essential for their victory and survival. Meaning that he knew it could mean the death of everyone under his command when he married Talisa, even if Walder Frey hadn't betrayed him, but had simply refused to help him any further as a consequence. He allowed himself to make catastrophic strategic mistakes based upon his feelings and at the expense of his banner men. Thats not what a responsible military commander would do. Second: He blindly trusted Roose Bolton, despite the fact that the Starks and the Boltons had been enemies and rivals for centuries. He should have expected that Roose would betray him and could be used against him by Tywin Lannister. And instead of trying to prevent this almost inevitable betrayal by charming Roose Bolton and offering him more power as a reward for his help once the war was over, he constantly pissed him of. That wasn't a very smart move to make, given the fact that the Boltons troops were second only to the Starks and a vital part of the norths armies. Third: Walder Frey was a pragmatist and a materialist, and obviously not a man of honor. The only reason he helped Robb was because he thought one of his daughters would become queen and his family would rule once the war was over. The moment Robb broke his oath, Tywin Lannister simply had more to offer to Walder Frey and he had more to gain from a Lannister victory than from a Stark victory. Robb should have known and foreseen that he would never forgive him and would definitely betray him. But instead, he was stupid enough to willingly follow the Freys invitation and lead his men into Walder Freys home, where they were at a disadvantage and to allow himself and his men to let their guard down and get drunk and defenseless. Come on. The Red Wedding was obvious from a mile away. Robb Stark was an idiot. A kind hearted idiot, but ultimately a dead idiot.