Interestingly, now Bruthor attacks after I release him with "feather drop" spell. The guards, despite being hacked to pieces by me, remain faithful allies...
this actually is a good thing. It's the only way to kill all the guards quickly without getting a global reputation hit.
featherfall Bruthor BEHIND you, cast slow on him, then attack the guards. so long as Bruthor is alive, you can attack the guards with impunity.
if Bruthor becomes a problem, blind or stun him or teleport him back into his cage
once the guards are all dead, you can deal with bruthor, and then free the other prisoners, who are grateful, but that's about it really.