There is another point against execution, once imprisonment is an options. This is not a pacificist argument. I'm not talking about self-defense or the defense, but cold-blooded execution after capture.
That is that to kill another human being always and everywhere harms the killer. So under no circumstances should we kill another if there is a reasonable alternative (such as imprisonment) - even given the risk that an imprisoned killer may someday escape, be released or kill again within his or her prison. Because the harm we do ourselves in killing another is too grave to inflict if we have a reasonable choice.
For people who are not already horribly damange, this is psychologically true - killing another person inflicts horrible trauma on the killer in most cases. This applies even in cases where it is absolutely justified. Note that this is not a pacificist argument. In cases of self defense, or defense of others the trauma still applies - but the harm in allowing murder to take place is worse. But once the murder is captured, and you are no longer in the heat of battle - literal or metaphorical, that argument no longer applies.
Of course that is merely the individual level. But I think execution adds to cruelty, callousness and a devaluation of human life at the social level too. The drunken frat boys I used to see holding parties to celebrate excutions when I lived in Texas strike me as a good example of that.
This is not to take away from Betsys argument that execution is also irrevocable; you can release an unjustly imprisoned human being; you cannot bring an unjustly executed one back to life. But I think it is almost as bad in the case of truly guilty people being executed - even the most horrible - the serial killers, the rape murdersers, the torturers. The horror here is the effect it has on all of us - the jury, the judge, the prosecutor, the guards, the executioners who all take part in the killing, and on the larger socieity. The tiny bit of extra safety we might gain, or the bit of satisfaction the blood revenge may give a few of us is not worth the damage it does to our souls, both individually and as a society.
t interesting x-post with Gus