Backbiting
As it is stated in one of the verses of the holy Qur'an (in Suratul Hujraat) and in some sayings of the prophet, a person backbiting someone is the same as he is eating the flesh of his dead brother. So if a Muslim is backbiting a non-Muslim does this law applies to him (the Muslim) because when the prophet say 'brother' he means a brother in Islam and a non-Muslim is not your brother religiously.