Well it does not matter now that he is dead, but if he were still alive I would not give him the death penalty. That is the easy way out, and Ariel Castro would suffer much more being locked in a jail cell for the rest of his life. Just like he did to those girls!
That will give him a taste of what it's like to be captive. Death is quick and only painful for a few moments, then snap, just like that Ariel Castro is going to be gone. Away from his worries, from the world who is angry at him, away from facing his own wrondoings, showing remorse, admitting his crimes fully.
In contrast, if he is in prison, he'll have to endure the uncomfortable conditions, plain food, being allowed little freedom, and other unpleasantries of cell life. While his soon to be life in jail cannot even compare with the atrocities that he commited and made the women suffer, at least he can get a taste of his own medicine, especially since he can't get visits from his anyone (not that they would want to). He would experience that feeling of never being able to be free, as I'm sure the women felt the same way, many times. Moreover, he actually can't get out, and he will never be able to get out-he's not going to live for 1,000 years, but considering he's still in his fifties, and healthy, he will be able to get a nice long taste of being captive. In fact, I think the law should restrict his life even more in prison, so he gets the full idea. And of course, considering he will die in prison, or prison custody, surrounded by the consequences of his crimes, then hopefully he will finally express the remorse and guilt that the women who suffered deserve to get from him.