Exactly. And that is a very important distinction. After 21 years he will have served his term. If afterwards he cannot be released into society he needs to be

Exactly. And that is a very important distinction. After 21 years he will have served his term. If afterwards he cannot be released into society he needs to be kept separate, but that is no longer a punishment. As such he will (or should) be treated well during this "safe keeping" following his sentence.