Here are some interesting remarks about that point from a poster on Jimmy Akins blog. I've often thought the same, but the poster, Deacon John Bresnahan, expressed the point very well:
Rarely do I ever hear mentioned that to keep some of the most vicious murderers in jail costs innocent lives. How many guards a year are murdered by escaping murderers who should have been executed? We had one recently here in anti-death penalty Ma. How many people in prison for crimes far lower than murder are murdered by murderers who should have been executed. We've had a series of them in Ma. including one this week. A few years ago in another state a young man in jail for a very short period (for drunkeness) was murdered by a murderer. Are anti-death penalty people willing to endorse creating jails where the imprisoned murderers are always chained, never have human contact or receive privileges of any kind the rest of their lives?? If such a jail were even proposed I can hear the howls and screams from the same people whose hearts bleed for murderers on the dp issue. Yet to keep those who are the most vicious class of murderers alive costs the lives of innocent people--by the hundreds over the years.This is a whole separate issue from whether the dp is a crime preventative in the wider society.Consequently it seems those states which have separate categories such as dp for those who are serial murderers or especially violent murderers and those who kill prison guards or inmates after already dodging the dp for an earlier killing--are handling the issue in the best and fairest manner and within traditional Christian ethics which especially values innocent human life but also is willing to accept the dp in definitely needed situations.
It is often stated as a fact beyond debate that modern technology can secure the safety of society at large as well as prisoners within the walls. I'd like to see some evidence of this. I've also never heard a convincing argument that the death penalty doesn't deter criminals.
Having said that, I think the death penalty is probably overused in our society and in the current politicized climate can be abused by unscrupulous prosecutors. If it's possible to secure the safety of society and the population inside the prison walls, then less drastic punishments should be used. I'm just not sure such a thing has been proven and am somewhat sceptical about the prospects. Just my opinion.
How many people in prison for crimes far lower than murder have killed prison guards?
With all due respect to Deacon Bresnahan, murderers aren't the only ones who kill prison guards. If he's going to use that argument, then he's also arguing for the execution of thieves, rapists, drug-dealers, and pretty much anyone in a maximum security prison.
I've also never heard a convincing argument that the death penalty doesn't deter criminals.
States such as California (which implements the death penalty) are seeing a horrendous uptick in violent crime between gangs (and sometimes involving innocent bystanders) in the L.A. area. Unless someone runs a Gallup poll of armed robbers and rapists, I don't know how one measures deterrance except by crime statistics, which are going in the opposite direction, if that's the case.