You cannot see inside a mans mind. Some people may legitimately believe a life in prison is preferable. So no i do not buy that argument personally, plus Norwegian prisons are a joke (running off that specific case).

The only valid one is the what if they were innocent. Personally, while that would be a tragedy, in most murders and rapes it is usually pretty clear. I'd stick the needle or pull the trigger myself. And I'd save everyone some money and do it quickly instead of keeping them on death row for ages.