game of thrones - When did the traitor turn?

Major spoiler for season 3 of GOT:

Lord Bolton betrays the Starks by returning Jamie Lannister to Kings Landing and then assisting in the massacre of the Starks by the Frays. In return, he is made Warden of the North (at least in the hypothetical Lannister victory version of the post war Westeros). Bolton had earlier served with distinction under Robb Stark, including leading the attack in which Jamie Lannister was captured in the first place. At what point did Bolton turn? How long had he been plotting with the Lannisters?

Answer

"When" is a good question. I think the answer is that Roose plots all the time. When Robb called the banners, I think Roose went through his options and thought about how to gain the most from the situation.

Spoilers ADWD below

We get some insight into Roose's mind through lady Dustin, who claims that Roose is somewhat of a psychopath, who has no feelings and likes to manipulate people for his own amusement. Even so, Roose likes to stay "below the radar", let other people take the blame while he reaps the benefits. Walder Frey certainly took most of the blame for the Red Wedding, for example, though my hunch is that Roose Bolton was the bigger culprit (he did personally kill Robb).

It is as Jaime said in ASOS (paraphrasing)-- that every lord has unruly bannermen who envy them their power. Tywin had the Tarbecks, the Tyrells have the Florents, Hoster Tully has Walder Frey, Ned Stark had Roose Bolton. Strength is the only thing that keeps such men in check. And when they smell weakness, they turn.

Spoilers ADWD below

Roose Bolton himself told Theon in ADWD that he had him to thank for winning the North. That house Stark was done when Theon sacked Winterfell. That was almost certainly not when he started plotting betrayal, but it would have encouraged him to set his plans in motion. Also, we do not know whether Roose and Ramsay planned to attack Winterfell, or if Ramsay did it on his own initiative and Roose felt compelled to see it through. We do know that Roose considers Ramsay his only option for house Bolton's survival (unless he lied to Theon about that in ADWD).

Roose found himself in possession of many important hostages, and the leader of big armies. Most notably, of course, he found Jaime which opened a dialogue with Tywin. He also got his hands on the Frey boys in Winterfell, which he no doubt used as leverage against Walder Frey. He sent Robett Glover and Ser Helman Tallart to raid Duskendale, where they were ambushed and defeated by Randyll Tarly and Gregor Clegane. No doubt this was done intentionally in order to kill off Stark loyalists. He did a few such manoeuvres.

It is interesting to note that Roose would never have believed he could take the north in his own name, he is too much of a realist for that: The North would never support Ramsay, or a Bolton, only a Stark. Which is why he could not be seen to be directly involved in the Red Wedding, and why fake Arya was so important to his plans.

So, as for when Roose started planning it, I think the answer is: He always planned it. It was just a question of when he could start doing something about it, when he was given opportunities and when Robb showed weakness.