It's fine if they kill each other. But is it possible to build a secure enough doorway that they can't kill the guards, then?
From what I can tell there is no penelty in having terrible prisons. So a great trick would be to intentionally try to get prisoners to escape and riot so you can continue making money.
It affects your prison's valuation. Which is basically like fortress wealth in DF. Time without incident gives you a bonus, unsolved murders, serious injuries and escapes reduce your valuation. Whether or not your prison valuation has anything to do with anything (one would think your monthly stipend would get a bonus from it or something) is unclear. Maybe it's just a "high score."
How viable is the dark ages prison - that being, you toss everyone into a brick room, lock the door, and put one booze-sodden guard in a wooden stool with a big brass keyring to make sure none of the bloodsplatter drips through the bars?
You can basically reduce a prisoner's standard of living to a 1x1 tile on a dirt floor with no light, laying unconscious in their own blood and filth, by virtue of you not providing them with shit so they riot continuously and then get thrown in solitary confinement. No showers, no rec room, no yard, no medical treatment, trudging pitifully along dirt and mud paths between barbed wire fences and tall concrete walls ....meanwhile your administrative staff sits in a massively overbuilt administrative complex surrounded by well manicured lawns and their offices are floored with nice hard wood. The warden might have solid marble tile in his office while the rest of the administrative complex is bedecked in mosaic and Florentine pattern tiles. Hell, I don't even know if prisoners
need to eat, they just want to.
So yeah, I'd say it's very possible to have a medieval style hell hole. Especially with an unlimited budget to hire new security guards the instant the old ones are KIA, who conveniently appear in the thick of a riot the instant they show up for work. Right now the only real danger, when cheating for money, is your prison getting set on fire. The other clincher is that you can't hire more staff than your daily stipend allows for their salary...so you're effectively capped on guards by prisoners.