I am completely in favor of the Death Penalty, but only when used under proper circumstances.
I believe much opposition to the Death Penalty results from two assumptions about the purpose of the Penal System. The first assumption is that criminals should be punished, regardless of rehabilitation. The second assumption is that imprisoning someone will prevent them from commiting further crimes. Both assumptions are false.
The first one justifies the anti-DP stance in which someone argues that a life-sentence is a "better punishment" of criminals because they "suffer" in prison for many years. However, punishment is irrelevant when the sole purpose is to exact revenge. Punishment is supposed to alter and improve behavior, not satisfy your hatred of a criminal. When considering this issue, we should put aside our petty desires and instead judge based on the ethics of the Death Penalty and the actual impact our treatment of criminals will have on Society.
This is where the second assumption comes into play. Most people treat the Death Penalty and a Life Sentence as if they have identical effets. However, that is not at all true. Criminals who are put in prison for life, by necessity, adapt to their new environment; metal bars and guards are not enough to stop a psychopath or rapist from acting out his desires furhter. When criminals are in prison, they can still commit crimes and perpetuate their own criminal attitudes and those of other individuals by collaborating and feeding off the criminality of one another.
Or in other words, puting someone who we believe can not be rehabilitated in prison will hurt those who still can be rehabilitated. The entire purpose of prison is to "save" criminals from their current behavior and teach them a lesson; allowing life-sentence prisoners to interact with anyone else and allowing the mixture of "hard" criminals with others only promotes the exact behavior prison is meant to eliminate. Murders, theft, rape, fraud, and violence all happen in prison still; our goal should be to prevent as much of that as reasonably possible.
Putting osmeone in prison for life may "remove" them, but it harms the rehabilitation of those who are in prison temporarily and does not prevent further criminal acts by the imprisoned. In some situations, the Death Penalty is necessary as the only solution to truly stop criminals who are so far beyond rehabilitation that they pose an extreme danger both physically and behaviorally to all those around them.
For example, we have this criminal named TERRY. He was put in prison for the murder of nine people in three families that lived along the same street. After he was in prison for a year, he attacked and severely injured a guard with his fists. He was punished further, but three months later he strangles his cellmate to death. After five years he has assaulted over a dozen guards and inmates and killed three other people in the prison. What should be done with him? Ethical restrictions placed on the Prison prevent permanent solitary confinement or other measures to keep him away from others.
In some situations, the Death Penalty becomes the only reasonable option, when an individual is so dangerous that the best course of action is simply a complete removal of the individual and any influence he or she may have on anyone. It is not a matter of "punishment" or "revenge," it is a situation of cold, hard reasoning about the damage an individual will cause if allowed to live.
I do believe ethical constraints on the Death Penalty should be strong. Very rarely should someone be punished with it, and even more rarely for behavior only performed before imprisonment. A man who simply stabs another person to death does not warrant the Death Penalty; a serial killer who has brutally tortured and raped a dozen individuals before murdering them and mutilating the corpses does deserve consideration of the Death Penalty (much like the BTK killer); an individual who committed a series of crimes including murder outside of prison and then stubbornly continues attempts to attack and murder multiple other people in prison also should be considered for the Death Penalty.
Above all, let this point get across: the Death Penalty should be allowed, but should also be reserved for truly extreme cases of criminality where effectiely no doubt at all exists about the impossibility of rehabilitating the individual.
Further, I believe any "ethical" rejections of the Death Penalty should be reconsidered in the light of Real Life: war kills, soldiers kill, criminals kill, and millions die every year at the hands of other individuals. If any person is justified in shooting another to protect himself or his nation, then that Nation should be justified in killing an individual who poses a grave, imminent threat to the citizenry of that Nation. It is not "different"; the killing that goes on regularly is the same as that proposed by the Death Penalty.
For any individual who believes the Death Penalty should be banned based on the ethics of killing a person, I charge you to hold to your belief in all situations and be a complete Pacifist, even at the stake of your own life, and actively against any and every form of killing. If you do hold true and reject violence, war, the martial arts and other combat styles, self-defense, euthanasia, abortion, medical life-support termination, and lay blame in accidental killings, then I respect your right to hold that position. However, if you believe any form of killing is in any way justifiable under any circumstnaces, your rejection of the Death Penalty is pure fluff.
Those are my beliefs about the entire situation, but for those readers who are too lazy to actually read a full argument, I will summarize it:
I support the existance and use of the Death Penalty in extreme conditions which warrant it, but not otherwise.