In my view, there should be three principles involved in determining the punishment of convicted criminals, in descending order.
1) Punishment. You did a bad thing and deserve to be punished.
2) Deterrent. You did a bad thing. We don't want others to do the same thing, so we are going to give everyone an example of why they shouldn't do it, by what we do to you.
3) Protecting the public. You have proven yourself a threat to other people, so we are going to protect society from you.
Peter Sutcliffe is challenging his punishment with regard to the last principle. But Peter, we haven't finished punishing you yet. We're still on point 1. Once that's done with, we can move on to point 2 and then 3.
Point 1 will be finished when you cease to breathe.
Point 2 will be finished when you're buried in the prison cemetery.
Then, and only then, the question of the threat you pose will become relevant.