Personally, I feel that prison is best used to give those with the potential to be rehabilitated a chance to learn from their mistakes (giving the benefit of the doubt wherever possible, as jail is a correctable error, whereas death is not).
However, for those who cannot safely reenter society, it may be necessary, for the safety of us all, to kill them. Doing so as "punishment" is wrong - it lowers us all to their level; however, when one shoots a rabid dog, one does so as a safety measure for everyone else, not in order to punish the dog for being rabid. Criminals should only be executed when there is no viable alternative, and such execution should be carried out with no more vengefulness than one would associate with killing a dangerous animal. (I also believe that incarcerating someone for the rest of their natural life is far more cruel than almost any method of execution one cares to imagine, because even the worst method of killing someone still winds up with them being dead, not stuck in a 10'x10' room for sixty or seventy years.)
You may notice that I refuse to cushion the situation in nice words there; we don't "put them down" for our safety, we kill them. We must be willing to face our actions, not hide from them. If you can't stomach the idea of killing someone, then for the love of sanity, don't fracking do it!! Just be aware of the possible consequences, should the next Jack the Ripper either escape or win parole by pretending to be rehabilitated. Either way, be willing to face it with your eyes open.
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.