It's like saying, "Imprisoning someone because they imprisoned someone when its illegal to imprison someone make no sense." But most wouldn't make that argument. Locke made a simple way around this: by having a death penalty, we are stating that life is so sacred that the only crime worthy of losing one's life is killing other people. ? Locke made this argument. ? By infringing on an individual's right to live, one has forfeited his right to live.
I think his point is the paradox of punishment - we ought to do (kill) what we ought not to do (kill).
The difference is that the wrongdoer deserves it, the victim did not.
And if a right is inalienable (is that in Locke?) then how can it be forfeited?
Psychopaths can't feel empathy or sympathy for other people, so I wonder how many (mass) murders or sadistic killers will sit in their cell or in isolation and actually regret their actions. ? If they're mentally or have been socialized to be incapable of recognizing other people's emotions and comprehending the true extent/nature of their actions, they basically get put in a cell and feel bad they go caught.
Are such people morally responsible for their actions?
Empathy/sympathy seems to be a part of being a moral agent.
So, if you don't have it, you're not a moral agent.
If you're not a moral agent, you're not morally responsible for your actions.
If you're not morally responible, you can't be justly punished.
So, we can't justly punish people that lack sympathy.