Firstly, I want to congratulating you people for not being like Iran or Punkistan or Afghanistan or Someother-istan. I hate these places and the sheer barbarity in them. Anyway ...
Regardless, it doesn't matter who you are. What matters is who the evils ones are, what are they like, what they do, the overall damage they do and such things.
So? We simply like to try to correct, instead of following emotional revenge feelings. even if correction does not always work.
1. Mistreatment of criminals and/or their execution doesn't necessarily comes from emotions. Some might do it as for the greater good of justice and/or as a deterrent for others for following the same trend. Capital punishments and/or other punishments could very well be for other purposes.
2. Just because they are [arguably] "following emotional revenge feelings", doesn't mean they are wrong. Their feelings could very well be justified and true, and the way they vent out their anger could also be justified and true.
3. Just because you guys like to correct people, doesn't necessarily mean that what you do is overall more constructive and beneficial in the long run as compared to what others do. You may be not like Iran, but that necessarily doesn't mean you guys are better.
We had a guy a few years ago who ran havoc and killed a multitude of innocents. It was in Norway. Yet, in Norway even life sentences are considered immoral. therefore, he will eventually get out of jail.
Okay, let's use this guy you are talking about in this case. Let's go by the criteria I invented in haste:
"It all depends on factors like --- The prisoner in question and/or her nature, the severity of her crimes, variables that played role in making that crime happen, and other important factors."
"It depends on how inherently evil the said prisoner is, how overall cruel and harmful his/her crimes were, the circumstances surrounding the prison, the prisoners actions, desires, thinking, habits etc.
An accurate enough value of all of this factors would be the criteria how the criminal is and treatment"
Now, let's do some approximations. Is this guy a hardened, habitual and inherently sadistic individual who habitually run havoc and kill innocent individuals? How severe was the damage done by this individual? What was his emotional states like? Was he under influence? Is he likely to commit such a crime again? Was he bullied and/or facing some agonizing situations, problem, crises etc.? What about the family members of the individuals he has harmed, did they lashed out at someone due to their anger at what happened to them and their loved ones? [AND MANY OTHERS FACTORS]
If the value we received from accurate enough calculations of these factors among other factors --- Capital punishment and/or slavery is overall is more constructive, beneficial to the society in general and also the individuals, and (arguably) taking the criminal out of the death row [and freeing him/her from execution], and into voluntary devotion to a cause that'll serve a greater good is also relatively less agonizing to the criminal.