There are circumstances where you need to keep someone in solitary, whether it's for their own safety or for the safety of others. You can't well put a hated prisoner in with the general population without expecting that they'll be murdered. On the other hand, you have prisoners who have made a living out of murdering prison guards.
What do you do with Thomas Silverstein[1]? Terry Nichols? Members of the mafia? Caught spies? They'd likely either kill or be killed. I don't really see a better option than isolating them. These aren't people who are there to be rehabilitated.