Of course, the Red Wedding isn’t just revenge for breaking an engagement. It’s also a play by Roose Bolton for his family to replace the Starks in the North, and an attempt by Tywin Lannister to win the war at a stroke. IIRC, wasn’t the actual Red Wedding plan itself more Roose Bolton’s than Walder Frey’s?

Mix of both. And yes, the Roose Bolton part is often ignored in discussions of Robb marrying Jeyne Westerling – Roose doesn’t give a damn about the Frey marriage, he wants the North. And between the two of them, Roose is the far more dangerous one, because Walder Frey is a bully and a coward. Roose is neither.