Amnesty International global report death sentences and executions 2020 [pdf]
amnesty.org.uk
amnesty.org.uk
https://www.irrawaddy.com/news/burma/myanmar-regime-hands-me...
That means that we cannot ham-handedly treat the execution of, say, a sadistic serial killer in the US the same way as we treat the butchering of religious minorities and political critics in China or Saudi Arabia. You must discern first whether the act committed is evil at all, then determine whether the person is culpable, then determine whether the death penalty is commensurate with the crime and the culpability of the person, and then determine whether it is prudent to administer said penalty. In the case of your typical serial killer, we know that murder is evil, we know that premeditated murder entails culpability and thus forfeit's one's right to live (pace the advocates of a squishy false compassion), and in most cases, the death penalty will likely be prudent, though at the very least we know it is merited in principle.
So there is no argument where the legitimacy of the death penalty in principle is concerned. And mercy would have no meaning if there was not something that is deserved in the first place. The debate is therefore in the sphere of prudential judgement. That is, is it prudential to exercise the death penalty under the prevailing conditions? Should we adopt broadly merciful legal norms w.r.t. crimes that deserve the death sentence, or should we leave it up to individual court decisions? Do political realities favor the exercise of the death penalty or abstention?
[0] https://www.ignatius.com/By-Man-Shall-His-Blood-Be-Shed-P166...