Wasn't there some kind of self-injury incident soon after his arrest? Banging his head against a wall, or something?
Leaving my professional hat firmly off, I think that he's probably got quite a strong sense of entitlement, and is probably completely disorientated by what has happened - "people like me don't get to account for our acts". I think that's going to fester away inside him, alongside the realisation of the total loss of control he has over his life, or anyone else (note all the manipulative attempts to get off the hook), and eventually the only thing he'll have left to control is his own mortality.
Professional hat back on, I do find myself wondering what happens to someone to make them become someone like this: to be able to cold-bloodedly, and in full public view, abduct a completely innocent person, and - intimately - commit those acts of violence upon them, without at any point during quite a long period of time stopping and wondering WTF he's doing...that's some pretty messed-up thinking.
We would do well to not just focus on retribution, but do some careful consideration about understanding people like this, if only so we can spot risky signs earlier and do something about them. And that extends to the ethos of the environment he worked in - how someone who was nicknamed "the rapist" and who got away with a number of sexual offences, despite that...these are things that need looking at deeply and urgently. Because he won't be the only person like that in the police force - I'd imagine that police work is very attractive to people like Couzens.