Some very good points made. And also some that I disagree with on so many levels.
My point, is that we are all paying our taxes to keep these people in prison. Some can never be released, so we keep them well fed and with a roof over their head, for the rest of their life - for free. So, whilst they have lost their liberty, they have also gained in some way. They never have to work again. They never have to worry about money again. They get 3 square meals a day. They don't pay tax. They contribute NOTHING. They commit a serious crime and then suck off the rest of us until they die. Our war veterans and our Pensioners don't get treated that well. 
Others are released, in the hope that they have rehabilitated. But some go on to kill again. Thanks for that. Thanks for releasing dangerous men back into the public arena. www.dailymail.co.uk/news/article-2420137/Revealed-The-12-convicted-murderers-freed-licence-kill-AGAIN-past-decade.html
Yes, I suppose we would sometimes get it wrong. I suspect that would be a minuscule number. Anyway, I did say, in cases where the crime had been proven without any doubt.
Of course some cases would not be worthy of death penalty - for eg. the battered woman that finally snaps and kills her partner. I wouldn't even put her away actually.
I wasn't condoning gory deaths. Just a lethal injection.
And...as law abiding, tax paying citizens, why should we all be put at risk every time a "rehabilitated" criminal is released, perhaps in your village or town. Maybe next door. Hey, maybe he'll date your daughter. Score.
Good job I'm not Prime Minister eh? I'd have the overcrowding problem solved over night tho! 