In the February 19 issue of America Magazine, page 5 includes an article on the death penalty. While I know the Catholic Church is adamantly opposed to the death penalty, in some cases a political state, or rather, a society of humanity has the need to defend itself from those who would want or choose to destroy that society.
On the other hand the death penalty ends a human life. Whether or not human life is sacred (it is), this ends the potential time that a person may "correct" their behavior; most prisons are under the codename "Department of Corrections". And if the person is guilty, as I'm sure the vast majority are, then I have a hard time thinking of a better penalty than letting the prisoner sit in prison and "think" about why they are there, what they have done, and to come to the resolution that they can't get out, get free, and perform a second crime.
I'd much rather have the prisoner come to the resolution that they are a bad person and that they are dangerous to society rather than just kill them. Yes, it is cheaper and faster, but its not having the Department of Corrections try to correct the individual.