We can start with "killing an innocent person is criminal"...

Isn't an act criminal simply by breaking a criminal statue? I.e. someone did something that we had already agreed shouldn't be done?

Criminality isn't absolute, it requires the context of what those in charge decided shouldn't be done.

What has innocent to do with it ?

Innocent under whose eyes ? one man’s revolutionary is another man’s terrorist. what morality and code stands the test of time ?

actions which was legal and moral say 200 years ago would be illegal and amoral now , who is to say how 200 years in future would see our framework for ethics from now ?

Taking any sentient life is a crime against nature and humanity.

Societies supporting death penalty are not civilized for the same reasons any cannibals are not.

Killing is killing, what difference does it make if you were hungry or felt scared or vengeance or merely sadistic to justify it .

I think... it has to do with "start with" part of their statement (as in where to start to find what different people can hopefully reach a consensus on), as a more-likely-to-be agreeable thing than the general "killing a person".

what good will consensus on a statement everyone already agrees on help with ? nobody is ok innocents with being executed[2] . Even the Texan criminal justice system responsible for a third of the about 1600 executions since 1976 will say they don't want innocent to be executed. They just say everyone is guilty.

The methods to establish innocence or exonerate[1] in legal systems objectively are extremely difficult or impossible. People will never be able to agree on who is innocent, no matter how obvious it looks to you or me.

DNA evidences have exonerated many in the recent years, many of the executed were mentally ill, a lot of the evidence is circumstantial and based on unreliable testimony which all too frequently is racially colored, there is no certainty they did the act they are accused of, let alone deserve to die for it.

The only meaningful consensus that can actually make a difference is that death penalty should be abolished.

It is barbaric and morally abhorrent,it is also not ineffective as a punishment, is not deterrent, and also extremely expensive.

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[1] Pardons is not one of them, pardons require admission of guilt

[2] Innocent Americans that is, we are quite fine with "collateral damage" of women and children without any due process in drone strikes or now in Gaza.

Criminals often have a series of codes or personal rules that guide them. It doesn't make them any less criminal.

It's only criminal because those in charge wrote criminal states declaring those acts as such. There's no universal, fundamental code that defines criminal or right and wrong.