John Stewart Mill's View

Mill explains that "There is not, I should think, any human infliction which makes an impression on the imagination so entirely out of proportion to its real severity as the punishment of death."  He also says "When there has been brought home to any one, by conclusive evidence, the greatest crime known to the law; and when the attendant circumstances suggest no palliation of the guilt, no hope that the culprit may even yet not be unworthy to live among mankind, nothing to make it probable that the crime was an exception to his general character rather than a consequence of it, then I confess it appears to me that to deprive the criminal of the life of which he has proved himself to be unworthy--solemnly to blot him out from the fellowship of mankind and from the catalogue of the living--is the most appropriate as it is certainly the most impressive, mode in which society can attach to so great a crime the penal consequences which for the security of life it is indispensable to annex to it."
In short--Mills believes that when a person commits a crime so great as murder with no evidence to suggest that it was necessary/could be somewhat pardoned, the death penalty is a punishment which he sees fit.  "...the most appropriate as it is certainly the most impressive..."

To force someone to cease to live, to exist, because of their horrific actions, is a justifiable punishment according to Mills.

Plato's Sage View of the Death Penalty

Plato would probably think that there are worse fates than death.  However, I'm not sure if he would be for it or against it, just that he would view it in a rational way.  His mentor, Socrates, was put to death.  He and Socrates' other friends and students begged for an alternate fine in opposition to death.  No such negotiation was followed through with though.  Plato would have agreed with the death penalty in the correct circumstances.  The reasons behind someone's sentence are what's up for debate, though, and that's when things get complicated.

Kant's perspective

Kant believed that it is wrong to punish people for utilitarian reasons.  If the core motive in punishing someone is to deter others, or to protect society, or to set an example, then the person punished is wronged and their humanity has been disrespected. Therefore, punishment must always be in response to guilt.

Equality is the principle that must be used in selecting a punishment. Kant uses a metaphor. He refers to the principle of equality as the one by which the pointer of the scale of justice is made to incline no more to one side than to the other.

Kant does NOT believe in the death penalty because he believes in respecting humanity at all costs.  He would view it as inhumane to end a life as a result of that life's mistakes.

Aristotle and the Death Penalty

Hey y'all (fellow philosophers)--
The death penalty is a very tricky topic.  However, Artistole would have had a pretty concrete opinion regarding it.  Aristotle would not oppose the death penalty, yet he would believe that in some cases you must take into account the background of the perpetrator.  He believes that people should get what they deserve, however, the justice cannot be neutral.  In conclusion, the death penalty should be given to those who deserve it in the eyes of Aristotle.