Lover or Fighter?
Robb does capture Jaime Lannister, and shows his intelligence by refusing to essentially “end the war” through one-on-one combat. After all, Jaime is notorious for being the “Kingslayer,” and also one of the best swordsmen around. So, Robb wisely keeps Jaime as a prisoner, and let him know about his incestuous secret: Joffrey is Jaime's bastard son, and not the rightful heir to the throne.
Meanwhile, Roose Bolton suggests torturing Jaime, but Robb understands that he must protect his sisters from harm. At this point, Robb begins to show his softer side, especially when he first meets Talisa on the battlefield. Catelyn reminds Robb about his debt to be paid to House Frey, and sees a telling look in her son’s eyes. Once again, Robb has been manipulated - this time by love. As his relationship with Talisa progresses, Catelyn realizes that the House Frey agreement has been compromised. She later releases Jaime Lannister to protect her daughters, and Robb begins a physical relationship with Talisa soon after.
Both Robb and Catelyn have good intentions, but their actions in Game of Thrones season 2 have deadly consequences. Robb’s secret marriage betrays the House Frey pact, and Catelyn’s decision to release Jaime angers Rickard Karstark, whose son was murdered by the "Kingslayer" during a previous escape attempt. This creates an internal dispute that cripples Robb’s army, and forces him into a desperate strategic alliance with House Frey, one that will go horribly wrong.
Pride Comes Before The Fall
Early on Game of Thrones season 3, Rickard lets Robb know that he’s not impressed with his decision-making. For one, Catelyn essentially disrespects Robb by allowing Jaime to leave; a sign of weakness for the King in the North. Furthermore, everybody sees that Robb has a romantic relationship with Talisa; another sign of weakness during a time of war. In fact, Rickard says, “I think you lost this war the day you married her.” In addition, Robb puts the war on hold to attend his grandfather’s funeral. Robb seems to be losing focus, according to Rickard.
Later, Robb’s uncle Edmure Tully attacks Stone Mill, which proves to a poor strategic move, despite a victory. Rob wants The Mountain to come their way. Instead, Edmure pushes him back. During the battle, the northerners capture Martyn and Willem Lannister - two kids - which essentially gives Rickard the opportunity to enact revenge upon Jaime and his family.
Rickard does indeed kill the Lannister children, which creates even more internal conflict for the northerners. Because Robb is a man of his word, and because he lives by a code (like his father, Ned), he believes that he should be the man to swing the sword. Meaning, it's he who must execute Rickard. Despite Catelyn’s urges to keep the man alive, Robb beheads him in public. The consequences are major. Robb loses almost half of his army; his pride seems to be more important than the big picture.
The Missing Piece, The Wrong Piece
Though Edmure makes some bad decisions, according to Robb, he’s right to believe that news of the Lannister deaths will spread. And after the Karstarks abandon Robb and the northern army, it’s obvious that Tywin will realize that the King in the North has become vulnerable. Behind the scenes, unknown to the viewer, a secret plan begins to form, one that will theoretically destroy House Stark.
At Robb's strategic table, he experiences an epiphany. A deal with House Frey will allow him to acquire more soldiers, and he will then attack Casterly Rock - the seat of House Lannister. Talisa likes the plan, and reveals that she’s pregnant. Everything seems to make sense now. Robb can own up to his marriage pact mistake, and re-pay his debt to House Frey. He’ll build up his army once again, and hit Lord Tywin where it hurts. Rob makes the proposal to House Frey, who responds with a counteroffer in which Edmure must marry one of Walder’s daughters. This is the context for the Red Wedding: the marriage between Rob’s uncle Edmure and Roslin Frey (who turns out to be quite beautiful).
So, Rob schedules a trip to The Twins, and Catelyn worries about their delayed arrival because of the weather. She seems to sense danger, fully aware that Walder isn’t the most open-minded gentleman. What Robb and Catelyn don’t realize is that their fate has already been sealed. The scheduled marriage is merely an ironic twist, one that will make Robb and Catelyn think about what could’ve been, if only for a brief moment before their violent deaths.
The Wrath of House Frey
The fateful Red Wedding episode begins with Robb and Catelyn strategizing. They both agree that everything will go according to plan, but only “if Walder Frey cooperates.” Catelyn is especially ready to take down House Lannister, and says “show them how it feels to lose what they love.” Curiously, this particular moment doesn’t transition to another Game of Thrones side-narrative. Even the narrative structure of “The Rains of Castamere” suggests that something is off.
During the wedding ceremony, a surprised Edmure sees his bride for the first time. In the audience, Walder looks at Robb and offers a creepy grin, with the implication being, in retrospect, that Robb would’ve had a perfectly acceptable bride if he hadn’t betrayed House Frey. In the moment, however, Walder's smile feels inconsequential because Robb is genuinely happy for his uncle, and excited for what’s to come.
Most importantly, Robb knows that there’s no threat to his safety. House Frey has executed the “Guest Right” gesture, an important part of northern culture. Robb may feel vulnerable when thinking about the larger picture, but at this particular moment, he’s full of pride and joy. Crucially, his beloved Grey Wind has been locked up outside. But why? And why are the doors closing during the post-wedding ceremony? And why is Roose Bolton not drinking? Unfortunately for Robb, Catelyn, Talisa, and Grey Wind (along with the entire northern army outside), the Red Wedding is merely a cover for a secret Lord Tywin-House Frey plan. Once again, Robb has been manipulated, and it costs him everything.