lets be serious here, Mental illness, perfectly sane, whatever the scenario, if you murder another human, cut their head off and eat them, you in no way shape or form will contribute to society in any beneficial way, nor should you.
Personally i believe humane painless lethal injection is the only method to ensure that an incident such as this does not happen again. The planet has quite enough humans already, bad apples like this one need to be removed.
Consider the victim, and any more potential victims, do their lives mean less than this man's?
*note i am not a capital punishment enthusiast. It should only be used in the most extreme cases i.e. Paul Bernardo, This guy etc.*