He wants to prove that it's perfectly possible to send an innocent man to the electric chair. That's their scenario, but in the film it will go entirely differently. In this film, there is all of Lang. The subject isn't really the death penalty. There might have been, but there are no longer.
Innocence is provisional, wanting proof is already guilt.
Dangerous for others. The journalist who struggles against the death penalty and the sadistic prosecutor who wants to apply it at all costs are brothers. One can expose an innocent man to be condemned to better prove himself innocent, the other is ready to condemn the innocent man. What they haven't expected is that the innocent man is already the guilty one. I have already said too much.