"What is the ruling of cursing the sinner of (the command of) Allaah in a certain matter, even if the sin was minor"?
Sinning is known (by) abandoning that which is obligatory (upon oneself) or doing that which is prohibited, and the sinner from amongst the Muslims is advised, and advising is obligatory upon every Muslim according to his ability.
The Messenger of Allaah (sal-Allaahu 'alayhe wa sallam) said:
((He who amongst you sees something abominable should modify it with the help of his hand; and if he has not strength enough to do it, then he should do it with his tongue; and if he has not strength enough to do it, (even) then he should (abhor it) from his heart and that is the least of faith.)), [transmitted by Muslim, 49]
and (Allaah) The Most High says:
{Help you one another in al-Birr and at-Taqwa (virtue, righteousness and piety)}, [Soorah al-Maa.idah, Aayah 2]
As for backbiting the Muslim, then that is prohibited even if he was a sinner, unless if someone sought your advice for (an issue related to) a transaction, then it is upon you to explain.
Shaykh Muhammad ibn Saalih al-Uthaymeen RahimaUllah
Mowsoo'atul-Ahkaam ash-Shar'iyyah - Volume 3, page 415