The Rambam writes in Sefer Hamitzvohs that the posuk that says v’naki v’tzadik al ta’harog is an issur to use circumstantial evidence in death penalty cases, even when its 100% clear from the evidence that he did. He says that the reason for this is that if we use circumstantial evidence then this will lead down a slippery slope where the evidence that we use will be less than 100% and then less than even that and that one day and innocent man will end up getting executed. He writes that its better for a thousand guilty to go free than to kill one innocent man.
Now the Rambam is talking about a system that demands that the court be positive that the person is guilty, and even so he says that we may come to use evidence that does not prove guilt 100% and take this evidence to show that it is 100% sure that he did it and that there is no doubt as to his guilt.
In America however the court does not have to find at all that they are positive that he did it. They don’t even have to find that they are “fairly certain” that he did it. The standard in american courts is that its “beyond a reasonable doubt” that he did it. Legal scholars say that this means about 75% sure.
So the issue with capital punishment in America is not whether a murdered should put to death, which is a mitzvah for bnai Noach.
The problem is that they put people to death even though they admit that they are not certain that he did it.
And the fact is that over the past 2 decades with DNA evidence it has been found that hundreds of people who were on death row did not do it. And had it not been for this evidence they would have been executed.
But the problem was more than just these cases where it has been proven that the person was innocent. There are cases upon cases where the prosecution withholds evidence, that is they had a witness that said that he saw the accused at another place at the time of the murder for example, and doesn’t tell the defense about it and so again an innocent man is sent to his death.
There are many cases in which the jury that convicted finds out that they had been misled by not hearing the full evidence and they write letters that had they known the facts they never would have convicted. But its extremely hard to overturn a jury verdict, and in almost all such cases the person is put to death.
There is one story that illustrates this kind of thing perfectly. A man was charged with killing a cop in a supermarket parking lot at night. The prosecution found this man had a .22 caliber gun and the cop was shot with a .22. They also provided a single witness, a woman who was standing 60 feet away and said that she could identify him because she saw him for no longer than 2 seconds. Based on this evidence he was sentenced to death.
Once he was on death row better lawyers started to look into his case. They found that the .22 that he had was proven by ballistics not to have been the murder weapon, a fact the prosecution didn’t tell the jury. They also identified other witnesses. 2 of them said that it was too dark to see the shooter’s face but that he was very tall, no less than 6’1. The person that was convicted was 5’6. The other 2 witnesses were both standing about 30 feet away, allot closer than the woman that identified him from 60 feet away and they said that from 30 feet it was impossible to tell.
The jury convicted based on the .22 gun that they were not told was not the murder weapon, and a single witness that was contradicted by 2 others that said he was too short and another 2 that said that it was too dark to see from even 30 feet.
When the jury heard of the full evidence they signed a letter saying that had they heard all of the evidence they would not have convicted and asked that the state not execute this man because of their verdict.
This case was in Texas and the man on death row appealed to then governor George W Bush to let the new evidence be heard. Bush refused and sent him to his death.
So the main issue with the death penalty is that they don’t require that there be proof that it is certain that he did it and also that the way the system is set up many innocent people are put to death. Allowing innocent people to be put to death, the way that Bush did, with a person that had 2 witnesses that said that he could not have done it, is murder itself.