I see somebody hasn't been watching The Walking Dead. Storing away the newly-turned doesn't end well. Sure, they may starve off, but that can take weeks, even months to happen. All the while, you'll be hearing their unending attempts to get out. Also assuming that they don't manage to break out in the first place. Then once they escape into your stronghold, you'd better hope everybody is already awake, lest you now have a completely overrun fortress.
Once they turn, they're already dead, and you didn't kill them. Re-killing them is simply giving them a dignified death.