so long as the prisoner must do all the labors to keep themselves and only get the space to do it in, i approve also.
killing in self defense is the only case where it is righteous. shooting a fleeing sniper, for example, would be legitimate, because if he gets away he can do it again with little impediment, and no recourse for the next victim either.
but if you have them cornered or cuffed there's no reason to kill. but they neither should be allowed to interact with civil society, and if they are violent while locked up as well, same thing applies, since their violence would impact the right of life of their fellow inmates.