I forgot about this one. Just wanted to come back to it.
That must have been one HELL of a probe. I mean, to execute all the prisoners that you've taken all the trouble to capture and hold for however long means that you must feel that you will not survive a second attack. I mean,the prison camp leaders abandoned their investment because there was a small attack that was repelled with minor losses?
Or did the PC's cream half the guards and then leave? Then again, the prison leaders had enough resources to toss around spells (why exactly did they have Magic Mouth spells prepared anyway? I mean, the PC's were only gone, what, 12 hours? 14 hours? The bad guys were so sure that they would escape that they prepped pointless taunting spells?), so, they must have had some pretty deep resources.
Yeah, this sort of thing is the reason I refuse to play with some DM's. Choo choo. Get on the railroad, never, ever deviate from the DM's carefully constructed plan for how you will deal with whatever problem du jour he's concocted and make sure that you never, ever, try to get ahead of the game.

No thanks. I prefer it when bad guys have realistic reactions and believable motivations.
DM's pulling out the punishment stick because we did something he or she didn't like is something I've long, long given up on enjoying.