"We all know that the great argument of those who defend capital punishment is the exemplary value of the punishment. Heads are cut off not only to punish but to intimidate, by a frightening example, any who might be tempted to imitate the guilty. Society is not taking revenge; it merely wants to forestall. It waves the head in the air so that potential murderers will see their fate and recoil from it.

This argument would be impressive if we were not obliged to note:
(1) that society itself does not believe in the exemplery value it talkes about;
(2) that there is no proof that the death penalty ever made a single murderer recoil when he had made up his mind, whereas clearly it had no effect but one of fascination on thousands of criminals;
(3) that, in other regards, it constitutes a repulsive example, the consequences of which cannot be foreseen.

To begin with society does not believe in what it says. If it really believed what it says, it would exhibit the heads. Society would give executions the benefit of the publicity it generally uses for national bond issues or new brands of drinks.

... How can a furtive assassination committed at night in a prison courtyard be exemplary? At most it serves the purpose of periodically informing the citizens that they will die if they happen to kill--a future that can be promised even to those who do not kill. For the penalty to be truly exemplary it must be frightening. [...] Today there is no spectacle, but only a penalty known to all by hearsay and, from time to time, the news of an execution dressed up in soothing phrases. How could a future criminal keep in mind, at the moment of his crime, a sanction that everyone tries to make more and more abstract?"

- Albert Camus, Reflections on the Guillotine (as transcribed by voctir)

Someone else quoting from the same:

"What then is capital punishment but the most premeditated of murders, to which no criminal's deed, however calculated it may be, can be compared? For there to be an equivalence, the death penalty would have to punish a criminal, who had warned his victim of the date at which he would inflict a horrible death on him, and who from that moment onward had confined him at his mercy for months. Such a monster is not encountered in private life."