I think if it’s impossible to be sure of innocence then you should not be possible to be sentenced to death, as one of the worst things a government can do is directly kill an innocent man. However there are some very clear cut cases (video+audio evidence and plea without coercion) where it should not matter and any execution that is quick and easy will do. I also agree with other people’s sentiment that firing squad can be more quick and painless and certainly cheaper than the shitshow that are lethal injections.