To counterpoint the OP:
1) I disagree. All laws and punishments are open to horrible abuse. We can't just get rid of them; we need better people making/enforcing them.
2) I disagree with your ends but not your point. The death penalty would be an effective deterrent if it were not allowed decades worth of appeals and such. However, for those who do not see it as a deterrent; if the threat of death doesn't stop you from raping and murdering your way through life, shouldn't you be killed anyway? I wouldn't want someone like that to continue to threaten society and quite possibly me and my loved ones.
3) I disagree. We are not implementing reasonable doubt anymore. We are too often resorting to unreasonable doubt. We let people whine and cry and get off for every reason. If we had quality people making and enforcing the law, innocent people would only be convicted if they were framed; which, let's face it, if someone goes to the trouble of framing you they aren't going to stop coming after you if you get out/off. We can't let our fear of punishing an innocent person wrongly get in the way of punishing a monster rightly. Which is better, to risk one innocent person's wrongful punishment, or to guarantee many innocent person's horrible violent murders?
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For the other points:
Overcrowding - Why should we continue to support someone who is a violent monster? Why should we feed, clothe, and care for murderers and rapists? They didn't care for their victims, and for us to care for the criminals is a profound insult to the victims' loved ones. These monsters have already destroyed innocent members of our society, why should they be allowed to continue to drain us? It's insulting that innocent homeless people starve while guilty monsters get fed in jail. No good will come of it.
Monster Genes - I saw this mentioned and someone else Godwined (sigh). I can't seriously believe that anyone would want a gene that causes people to rape and murder to stay in our gene pool. I shouldn't have to explain this or the history of the Nazis. If genetic engineering provides a cure to those who have mental diseases which cause them to commit such horrors, isn't that cure a betterment of society? Less innocents will be brutalized, and people who aren't in control will be again - and will stay out of jail. Maybe we could even cure sociopathy this way. How is this in any way bad?
Rehabilitation - I agree, not everyone is a violent monster and a threat in general. But that should be evident enough during the trial. If this is your 3rd murder/rape this month, or your 23rd case overall, you obviously are either incapable or unwilling to stop. You deserve death so the rest of us can be safe from you.
Humaneness - This is total horseshit. If you are a violent monster, you have been horribly inhumane to your victims; why in the hell should we be humane to you? You have no right to demand such treatment; and again, it would be a horrible insult to the victims' loved ones. Not because the victims' loved ones are bloodthirsty. It's just that we shouldn't treat the criminals better than the victims. Rorschach said it best: "I was too soft on criminals - I let them live."
Cheap labor - This is for criminals who aren't monsters. We don't have to worry about them turning on us or escaping being a danger to society. If a thief turns on you, it's doubtful he'll kill you. And if a thief escapes, he'll just steal stuff, which is replaceable. Thieves being in and out of jail doesn't bother me, because they are paying for what they've done, and everyone's okay otherwise. Stealing stuff is only worth a certain amount of recompense. Thieves may be risks, but they aren't threats/dangers. Theives don't rape and murder. And if they do, they stop being theives and start being monsters. It's that simple.
The only crimes the death penalty should apply to are intenionally letting someone innocent die (e.g. treason leading to a terrorist attack, drunk driving, etc.), extreme physical violence/murder (murder isn't the only thing you can't recover from), and any sexual violence/child abuse. Basically anything which ruins or ends an innocent person's life. Ironically enough, despite my dislike of Barack Obama, he did make a good point when he said that the death penalty was appropriate when "the community is justified in expressing the full measure of it's outrage."
If it's obvious that you are a violent monster and a threat to society, you should be killed to keep us good people safe.
It's simple - dead monsters don't commit crimes.