Question Summary:
I know that it is extremely sinful to backbite on other muslims. But is it also sinful to backbite on non-muslims as well?

Question Detail:

I know that it is extremely sinful to backbite on other muslims. But is it also sinful to backbite on non-muslims as well? 

Answer :

In the name of Allah, Most Gracious, Most Merciful

Assalaamu `alaykum waRahmatullahi Wabarakatuh
 
As it is sinful to backbite a Muslim, so too is it sinful to backbite a non- Muslim. However, if due to some reason a person has evil qualities or incorrect beliefs that would cause harm to others, then to forewarn people to protect them from such evil and wrong belief will not be considered as backbiting.
وإذا كان الرجل يصوم ويصلي ويضر الاس بيده ولسانه فذكره بما فيه ليس بغيبة حتى لو أخبر السلطان بذلك ليزجره لا إثم عليه وقالوا إن علم أن أباه يقدر على منعه أعلمه ولو بكتابة وإلا لا كي لا تقع العداوة
 وتمامه في الدرر ( وكذا ) لا إثم عليه ( لو ذكر مساوىء أخيه على وجه الاهتمام لا يكون غيبة إنما الغيبة أن يذكر على وجه الغضب يريد السب
 
(Durrul Mukhtaar p309 vol.6)
 
And Allah knows best
Wassalamu Alaikum
Ml. Zakariyya bin Ahmed,
Student Darul Iftaa
Checked and Approved by:
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In'aamiyyah