When Roose freed Jaime, Jaime said jokingly, Give my regards to Robb Stark. It was him just running his mouth, it didn't really mean anything. Roose however actually repeats this when he betrays Robb. It was him being cruel, Yes I had Jaime and let him go, yes I sided with the Lannisters. The unfortunate part of this though is that Catelyn having freed Jaime personally, hears this remarks and dies thinking he was behind The Red Wedding. When she returns as Lady Stoneheart she surely remembers it. And poor Jaime, having worked so hard to redeem himself, can very well be done in by something he likely doesn't even remember saying, and surely meant nothing by. But it's basically his chickens coming home to roost, he's put out do much negativity that even though now that he's changed that negativity is bound to be his undoing. It's one of those things. Forget your jacket, catch a cold, and somebody calls you Snot Boogie, and your Snot Boogie for life.