I am with Michael and GreenGoddess on this one.

The miscarriage of justice risk is a no-brainer, really. In the case of the likes of Peter Sutcliffe and Harold Shipman: Death is far, far too good for 'em.

And additionally, with regard to...

"Genuine impulse crimes where you could say probably did not mean to kill"...

I am curious: What would you have done with the likes of Emma Humphreys, Sara Thornton, Kiranjit Ahluwalia and others who killed after years of domestic violence, bearing in mind that some of them were considered unsavoury characters at the time of their original trials (and therefore 'obviously guilty'), and another was convicted despite the introduction of the provocation defence and knowledge of her history of abuse, due to the evidence of an 'expert witness' that she was "remarkably resilient"?