Gabriel, I stand by my original interpretation. Although you are right that the “not at all” bit sounds strange, it makes perfect sense in the light of what Epictetus says immediately afterwards:
“What you should be asking instead is this: ‘This man who has fallen into error and is mistaken about the most important matters, and thus has gone blind, not with regard to the eyesight that distinguishes white from black, but with regard to the judgment that distinguishes good from bad — should someone like this be put to death? If you put the question in that way you’ll recognize the inhumanity of the thought that you’re expressing, and see that it is equivalent to saying, ‘Should this blind man, then, or that deaf one, be put to death?’” (I.18.5–7)
So, “not at all” refers, I think, to the fact that “should we put this thief to death?” is the wrong question to ask. Thanks for pushing me to go back and check.