[X] "And in the darkness of the Black Cells, I have seen a vision. No matter what I say today, the 'King' here will kill me as a traitor. But he will rue it sorely, for the young wolf has come down from the north, to run the rivers red with blood. He will kill and capture dogs and roosters and boars and unicorns, and capture three lions as well... only the old wolf will be able to stay his wrath..."
Joffrey's weak point is his ego. He wants to show he's in control of the situation - that's why he killed Ned, to show his mother and Sansa no one can control him. We say he'll do it, he might change his mind just to spite us. Furthermore, for the moment we appear a bit insane, and killing someone insane isn't interesting. Keeping them alive and making fools of them is far more fun (see Ser Dontos). And if we're alive when Kevan gets here, we probably manage to survive.
On the other side, if Joffrey does kill us nevertheless, we'll grt hailed as a posthumous prophet, demoralizing the Lannisters and possibly boosting Stark sympathy and popularity