(Forewarning, posting while tired. I'm likely overlooking something, and may not be as coherent as I'd like.)
I believe that if we took the death penalty seriously as a society, and actually used it, it would stop being an empty threat
Let's say that for the sake of the argument the only ones deserving the death penalty are those who kill other people.
Oversimplified, but I see no need to debate the point.
And let's discard those who have done so by accident. We just want the people who have done that on purpose.
Who kills another person on purpose? As a civilian, not employed by the government, in peace time, in self defense, not trying to prevent someone else to commit murder...
Who are the premeditated murderers?
You got two groups. Mentally deranged people and criminals.
Those are excessively large and diverse groups. Considering how "criminal" tends to be defined, it may be a tautology to split them this way, but I'll accept it for the time being.
Now... Mentally deranged people are mentally ill. THAT is the reason they commit murders.
Giving them the death penalty is basically killing people for being sick.
For those who kill only because they are mentally deranged. The distinction is a hard one to make, and I don't think it can be made in most cases. Let's say the severely mentally ill don't qualify, and move on.
Also, do you really believe that the insane person will take heed of the threat of death penalty?
Insanity isn't boolean. It isn't all or nothing. It is a continuum (in the very least). It depends on the degree of insanity. Some of them will, yes. Even if mental illness is off the table, the knowledge that death penalty is practiced on murderers will deter some of the insane. Remember, they've got a screw loose. Most of them don't want to admit that they're crazy, and the insanity defense won't always occur to them during pre-meditation. To premeditate the insanity defense, you would need to admit to yourself that you're cuckoo.
Either being with a long history of mental illness or just cracking and loosing it for a moment under the influence of stress, drugs or whatnot.
Drugs are not a good excuse. I'd need more information before I'm willing to decide if they are an excuse at all.
Snapping under stress? "Jim made me stressful, but I killed him. Now that he's gone, I'm all better. Honest!" Right... Now, there are cases of longstanding, undiagnosed mental illness. Add an unreasonable stressor, and things sometimes go from potentially dangerous, to deadly. These are two very different things. We need to be really careful about anybody we permit to take the insanity defense. It should be quite hard to get out of the psych ward after killing someone.
But again, remember, crazy isn't an absolute. We need to make an honest assessment of potential for premeditation and potential for self-restraint. Only when one of those is at an exceptionally low level (extreme clinical) should we just accept that they couldn't help themselves.
Some of them even believe that they are doing god's work and that there are really good things waiting for them if they martyr themselves.
That group is exceptionally scary. They will either attempt suicide, or they will be especially motivated to try to seem cured, to get back outside and kill again.
So, we're left with the other group - criminals.
A wide and varied group.
The kind of people who's "job description" involves "every day you may be shot and killed by police, your friends, your competition, family members and many other people not listed above".
For some of them, it's a job. For most of them, it's a hobby, or a lifestyle. Most criminals aren't "career" criminals in the sense that they make their living that way. Most criminals don't spend 8 hours a day in illicit activity, or fear that a cop/rival is going to make a move on them at any time. Many of them do, sure, but not most.
Even most of those who do make it their occupation don't live their lives in constant fear of being gunned down. They wouldn't chose that kind of life unless it was relatively safe, and made good money.
Some gangs fall into your description, sure, but even most gangs don't. Most gang members know somebody who went through the court system and was eventually released. Most gang members don't know someone who was gunned down by cops. (just for instance)
So, you're threatening the people who are already living each day expecting to be killed - with killing them unless they are killed first by almost everything and everyone in their life.
Those who actually fall into this category? They won't be as deterred as other criminals, true.
Where's the deterrence factor then? Who is being deterred?
Criminals who'd rather "do a nickle" than get the needle. Even those who enjoy murder are likely to think twice... and occasionally chose a different option.
Let's not forget one group that we've overlooked so far: crimes of passion. In other words, people who have anger problems. I'm not talking about acts "in the moment", but where someone leaves angry, and comes back with a gun (etc). Some of those can be deterred by the thought of capitol punishment.
As for prisons being unpleasant... there is no need nor value from that.
People who leave prison frequently, emphatically declare that they're not coming back. They'll say it to everyone and anyone. They mean it. This is insufficient, as many of them do come back, but they have a desire to stay on the straight and narrow. That is a starting point.
I'd much rather have the criminals be reformed and taught to control their impulses while being taught how to get out of the life of crime than being trained to be "harder".
Uh, I'm not sure you really read what I wrote. The average prison experience is the wrong experience. The ideal prison experience would still be unpleasant, but it wouldn't be dangerous, and it wouldn't be designed to ingrain machismo and other "hardened" behavior.
One thing that we have learned about reform, it must be the prisoner's choice. Some parrallels can be drawn to drug addiction. If they don't want to change, no amount of "reform" is going to take. Reform should be offered to them. It should be made abundantly available, but we can't coerce reform. We can only encourage them to take it.
As for giving the prisoner the choice, you can't have that on account that the death penalty is punishment.
You can't have the prisoner making the choice cause that would be like letting him/her commit suicide.
And suicide, in the mentally deranged world where the death penalty is the remnant from the time when it was viewed as sending someone to be judged by a "higher power" than earthly laws (which is why they get priests and whatnot) - is both a sin AND the prisoner escaping prescribed punishment.
In the world that forgoes on the "sending them to god to be judged" bit, it's simply escaping the prescribed punishment.
I.e. Red tape. It has to be done by the book.
I don't believe that punishment is the rightful place of government, strictly speaking. Anything the government does (including "punishment") should be to prevent future crime in some way. Vengeance has no place in the courtroom. (theoretically)
Not just to kill the prisoner but to make sure that he/she is really dead or some may try to game the system.
Easy enough. Once they're dead, and have been for some time, you cut the spinal cord. Blowfish toxin won't help you recover from that. Besides, it will be pretty obvious when most prisoners are trying something like this.
And that's without going into the whole "cruel and unusual" thing.
"Unusual" is easy. That just requires an even application of legal principles. One starts with stare decisis, and moves from there to even-handedness between different laws.
"Cruel", on the other hand, is open for debate. I'll point out that fines are one degree of cruelty. House arrest and Incarceration are higher degrees. Capitol punishment is an even higher degree. It would be more accurate to ask if it is a disproportionate degree of cruelty.