I used to support the death penalty, but as I get older, it's just much easier to see that apart from the primeval instincts that make us revengeful and leave us willing to retribute after someone has done us wrong, there's nothing that justifies it. Ergo, the death penalty is irrational.
The cost of a thorough judiciary process and all the expenses connected with it are significantly higher than the potential gain of killing the perpetrator.
If the costs of the judiciary process are decreased to a minimum, the proportion of innocent people executed rises to ridiculous levels. There's plenty of examples of that in countries like China or Russia. Just to give you an example - 6 people were executed for the crimes of a single mass murderer http://en.wikipedia.org/wiki/Andrei_Chikatilo
It makes much more sense to keep mass murderers etc. imprisoned until they die and to release all first-time offenders after a limited amount of time (including murderers), because the reoffending rate for murderers is very low.