Yeah, I got that far. Problem is though, the guards will start hammering down on the prisoners.
I ended up giving all guards a single bar of terebrium and waited until they were dead.
Unfortunately they didn't die but instead remained alive with 1 hitpoint.
Was easy enough to finish them off one by one though. And because I could instantly kill them, they didn't have the chance to bring down my reputation. :S
Anyway, I am fairly sure this is a bug that needs fixing.