Am I sinful if I backbite a non-Muslim?

Mentioning what he has made open of his disbelief or his sin is not the forbidden backbiting. Imam Ibn Baz said the disbeliever has no protected honour in what he has made open, and likewise the sinner in what he has made open, and it may even become obligatory when there is a need for it. As for what he conceals, the scholars differed.