You are looking for retribution not justice. “an eye for an eye makes the whole world blind”.
Retribution is justice.
The world isn’t fair. Good people are starving, individuals are sitting in prison because they sold a harmless plant, women are forced to have children because they live in the wrong state or country, countless Americans go into crippling debt because they don’t work for an employer that gives them good medical insurance. These people deserve help, but in many (most?) cases, they’re not going to get it. Many of these people are “good” people, but life hasn’t given them what they deserve.
On the other hand, you have Kenneth Smith who murders a woman and ends up on death row. He does not deserve the same human rights afforded to people who treat their fellow humans with kindness. Why should he have a peaceful death; his victim didn’t. A prolonged and painful death is fitting to his crime, and it’s what he deserves based on his actions. It doesn’t offset the suffering of others who don’t deserve to suffer, but it helps to restore the balance.
I strongly disagree with the idea of “an eye for an eye makes the whole world blind.” That assumes that the individual who’s serving justice (the executioner in this case) is doing something wrong. They’re not; they are ensuring that someone receives the treatment that they deserve. Justice is served, and there’s no “eye for an eye” to give in this case.
We may never agree on this, but I do not believe that my thinking is underdeveloped or immature. People who live a life that doesn’t cause harm to others should have every advantage in the world that they can; if I can do something in my own life to help achieve this, then I’m certainly willing to. People who live a life that causes pain and suffering to others should receive the same treatment, and while it’s not in my power to make this happen, I’m certainly happy that others can.