a guilty, condemned individual perhaps is lacking a measure of humanity, but aren't we also showing that same lack of humanity by taking the condemned's life? Is it not more human, to be at least somewhat merciful and spare his/her life? Again, the practical side is it is just not worth the risk to those wrongly convicted to employ the death penalty.
There is an argument to be made that incarcerating someone for their entire life is worth than death. I hope not to personally know the answer to that.