That's not correct, and tells something about your preconception. I was told by someone who had been to prison that those doing time for murder or manslaughter are considered less dangerous, they killed their target and that was that. The ones in there for GBH, those are the ones you have to look out for, they have low impulse control and low intelligence and are basically incorrigible.
From a policy POV, the comment is entirely serious and rational, appropriate.
You’re stating a moral axiom, one that is widely and increasingly rejected in civilised countries.
Assuming you mean “capital punishment for intentional murderers” then how do you defend this? A murderer kills to satisfy some need, you appear to wish to death on someone to satisfy some need of your own ie “justice”, even presumably when you aren’t personally victimised by their crime.
It seems capital punishment creates at least two evils - an unnecessary taking of human life, and the encouragement of sadistic ideation in the general population.
Taking a life can be a necessary evil. In "civilized countries" as how you put it, the next of kin has the choice on whether to forgive the killer or take blood money, or have him executed by the court. This is for one on one incidents. For other cases, such as serial rapists or serial killers, the decision is 100% in the hands of the court, as it is way bigger than a single interaction.
Regarding civilised or not, perhaps this might be of interest:
https://goodessaysblog.wordpress.com/2016/10/23/capital-puni...
> but that isn’t the case in capital punishment as someone in custody is no longer a mortal threat to society.
It's not just about being a threat, it's about justice.