That is such a push button topic. For me, the only time or the only circumstance that I am in favor of the death penalty is if there is a chance that that person will get out and do whatever they were doing again.
That said, condoning killing is one of those things we feel is wrong as a society, yet we have wars all over the place and so many feel that killing in that circumstance is okay.
In the US, many states still have the death penalty and to actually fulfill it cost way more money than keeping that person in prison for the rest of their life - that money could be spent on healthcare for kids.
Your idea of testing is interesting, but knowing that as I am writing this, there are quite a few innocent people sitting on death row, I don't think it would be a good idea.
As long as people are doing the judging, there are always mistakes, biases, the real quilty being rich and framing a poor person, that the risk of impacting an actual innocent person is too big.
Being for war and against the death penalty is hypocrisy, I agree. This idea would only be put to action in a proper and good way if the criminals would be given a choice to participate in testing. I am sure that many of them would say yes to it and use it as a way of giving something good back to the world. Thank you for sharing your views on this.