We'd then also likely be killing 50-100 innocent people a year.
https://www.law.umich.edu/special/exoneration/Pages/Exonerations-in-the-United-States-Map.aspx
https://www.law.umich.edu/special/exoneration/Pages/browse.aspx
As I've said before here, I'm 100% for the death penalty, but only with multiple, independent, absolutely concrete sources of evidence. Or the murderer freely confessing (like in a selfie video with a bodycam during the murder, not "He confessed during interrogation").
Otherwise, things like lying witnesses occur at an appalling rate (see the link above). How many times have we angrily discussed here, some innocent college kid getting railroaded for rape because a girl was mad at him?
Edit: corrected my number estimates.
I respect that argument. I think it is by far the best argument against the death penalty.
I do not agree with it however.
Punishing criminals is by far the most basic, undeniable function of legitimate government through all history and in every major religious tradition that I am aware of.
Every human action causes harm and death to some individuals- medicine, transportation, policing, electicity generation and so.
The question is not "Will this ever cause a human being to die?" but "is it worth doing despite the fact that it may kill an innocent person?"
For example, I have read that, on average, 390 children die yearly from drowning in swimming pools and spas in the USA.
That is about four times the maximum number of potential innocent deaths you attributed to the death penalty.
I do not think it follows that, because hundreds of children die yearly from drowning in home swimming pools, that home swimming pools should be banned by law.
That is because there a benefits to swimming pools- healthy excessive, fun, and teaching people how to swim, and so on. It is actually possible that home pools decrease drowning deaths be teaching children to swim.
In the same manner, I want you to consider the benefits of killing murderers. Dead murderers can no longer kill guards and other prisoners (which happens a lot in the usa- the murder rate of prisoners in the USA is about 5 per 100k yearly). They can no longer get on parole and murder others. They can't escape and murder others. Their deaths will, in my opinion, deter other potential murderers (at least if the death penalty were swiftly and universally applied to convicted killers). I believe very strongly that a quick, universally applied death penalty would decrease the murder rate. I could be wrong. I don't think so though.
Another benefit of killing murderers is that they are evil people who deserve to die. And it is the most basic function of the state to kill them. It is time for the state to start doing its job and get rid of these people.