How was Kanbaru's contract made void? I just finished episode 8 of the Bakemonogatari series, and I'm slightly confused about how Kanbaru was saved. If I'm not mistaken, Oshino states that there are two ways to save Kanbaru:


Cutting off her arm
Somehow making the contract with the rainy devil unachievable and void


Clearly the second option was taken, but I'm not sure how the appearance of Senjougahara and her subsequent actions made the contract void. It seems clear to me that the "voidness" of the contract stems from the fact that Kanbaru wasn't able to actually permanently kill Araragi. However, I'm not quite sure how doing so would violate the terms of her contract in light of Senjouhara's comment to the effect of "I would kill you if you killed Araragi". Is this simply because there are also some other "terms" to the contract (e.g. Kanbaru has to be alive) or is this perhaps because both her "conscious" (to be with Senjougahara) and "unconscious" (to kill Araragi) wishes have to be taken into account?
One way to explain this would be that it's impossible for the Rainy Devil (possessed Kanbaru) to kill Araragi, and so the contract is automatically impossible to complete. Had the Rainy Devil known that, it wouldn't've agreed with such condition. But because it seems too simple and there are many things hinting that this wasn't the deal breaker, here's another way to explain this:

The whole deal with the Rainy Devil is that it's a devious creature and will try to fulfill its owner's wish as deviously as possible. Remember how the first time Kanbaru wished to be faster than her classmates? Rainy Devil beat 'em up so they couldn't compete. Now guess what Kanbaru wished for concerning Senjougahara.

Because Kanbaru wished to be close friends with Hitagi and then suddenly Araragi appeared on the horizon, becoming Hitagi's boyfriend, the Rainy Devil decided that in order to gain Senjougahara's love (and let's not forget exactly how much Kanbaru prefers girls over boys), Araragi would have to be removed from the picture. By death, of course, because it's Rainy Devil.

So after Senjougahara waltzed in on Rainy Devil and Araragi's one-sided battle and explained that if Koyomi were to die, she'd never stop until Kanbaru, who caused all this, was dead, which would make Kanbaru's wish to become close friends with Senjougahara impossible to be fullfilled by killing Koyomi.