What is the rule on calling fellow muslim 'Mujrim'

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

First of all, it should be known that a Muslim is free of fault by default, and accusing one of anything without evidence is forbidden. Allaah says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.} [Quran 49:12]

Describing a Muslim as being a criminal is a clear and offensive insult. If it is attributed to someone who is innocent in his absence, then it is slander, and if it is in his presence, then it is an offense, an obscenity, defamation and transgression.

The Prophet, sallallaahu ‘alayhi wa sallam, said: “Indeed the believer is not a slanderer, one who curses frequently, one who indulges in obscenity, or one who engages in indecent talk.” [Ahmad] Also, the Prophet, sallallaahu ‘alayhi wa sallam, said: “Insulting a Muslim is Fusooq (disobedience).” [Al-Bukhari and Muslim]

If the person who was described as being a criminal deserves such a description because of a crime or offense he has committed, then the person who comes to know about his crime is principally required to disapprove of his act and to conceal his sin and not propagate it. It was stated in the story of a Maa’iz that the Prophet, sallallaahu ‘alayhi wa sallam, said to a man from the tribe of Aslam whose name was Hazzaal (who ordered Maa'iz to go to the Prophet, sallallaahu ‘alayhi wa sallam, and confess that he had committed Zina): “O Hazzaal, if you had covered him with your garment, it would have been better for you.” [Maalik, An-Nasaa’i, and At-Tabaraani]

This is in regard to someone who does not commit the sin openly. As regards someone who commits sin openly, it is permissible to publicly condemn his sin so that he might be deterred. Ibn Hajar said: “An-Nawawi said that whoever commits disobedience openly or practices a religious innovation openly, it is permissible to mention what he has committed openly without mentioning the sins one commits in secret. Also, An-Nawawi said in his commentary on Saheeh Muslim in his explanation of the Hadeeth that reads: “What an evil brother of his tribe...”: “This Hadeeth indicates that it is permissible to treat someone kindly in order to avoid his harm, and it also indicates the permissibility of backbiting a disobedient person who openly commits sins, and the one whom people need to be warned against.”

Allaah Knows best.