There is no such thing as humane capital punishment. No person or authority has the right to kill another human being, no matter how serious their crime or mental state. Two wrongs never, ever, add together to make a right.
As a clearly dangerous individual, Robert Maudsley should be placed in a situation where he cannot endanger the public, other inmates, prison staff and society in general. If the best means of doing this is the creation of a special unit for criminals such as him, then that should be done, and our society should be prepared to pay for it.