A few years ago, the Beeb broadcast a very interesting series on BBC Two, about the rise of the Third Reich in Germany in the 1930's. It made it very clear that the Authorities "practised" genocide on the mentally ill, long before they rolled it out for the Jews, Gypsies, Communists, Homosexuals, etc.
They tried various ways of murdering mental defectives, but one method that seemed to work was to herd them into a large shed or barn, seal that place airtight, and then feed the exhaust of a large truck into the place. This resulted in the death of those inside by carbon monoxide poisoning.
That sort of thing was disgusting and completely out of order and is universally recognised as such.
Once we start to decide that some people are worth keeping alive and some people are not, is the first step down that slippery slope to complete and utter barbarism.
My heart goes out to Rigby's family but murdering his murders isn't going to bring him back. It is also highly unlikely that their execution would prevent future events of a similar fashion. In fact, it would appear that it might actually encourage further outrages, as it seems that these seriously misguided Muslim terrorists look upon death as some kind of reward for their activities.