In the name of Allah, the Gracious, the Merciful
It is not permissible for a Muslim without judicial authority to implement Islamic legal punishments on his or her own initiative. Vigilante justice, or “taking the law into your own hands,” is forbidden in Islam because it leads to social chaos and likely causes even greater harm than the original crimes. In the same manner, it is not permissible for a Muslim to declare war or to commit an act of war without lawful authority.
Abu Huraira reported: Sa’d ibn ‘Ubadah said, “O Messenger of Allah, if I find another man with my wife, should I leave him alone until I bring four witnesses?” The Prophet said yes. Sa’d said, “Never! By the one who sent you with the truth, if that happened to me I would quickly grab the sword!” The Prophet, peace and blessings be upon him, said:
اسْمَعُوا إِلَى مَا يَقُولُ سَيِّدُكُمْ إِنَّهُ لَغَيُورٌ وَأَنَا أَغْيَرُ مِنْهُ وَاللَّهُ أَغْيَرُ مِنِّي
Listen to what your leader says. Verily, he has a sense of honor, and I have a greater sense of honor than him, and Allah has a greater sense of honor than me.
Source: Ṣaḥīḥ Muslim 1498, Grade: Sahih
In this example, the Prophet (ṣ) has forbidden a man who witnesses his wife committing adultery from implementing the legal punishment or retaliating against her by his own initiative, that is, without due process. The Prophet (ṣ) also informed the man that his sense of ‘honor’ or jealousy (al-ghirah) is no justification for such vigilantism. Hence, the concept of ‘honor killings,’ or extrajudicial acts of murder motivated by a sense of tribal honor, have absolutely no basis in Islam.
The unanimously established rule in Islam is that legal punishments may only be ordered by properly authorized judges, who must follow due process in the interest of fairness and apply mitigating factors to protect innocent people and prevent judicial abuse.
Ibn Muflih writes:
تَحْرُمُ إقَامَةُ حَدٍّ إلَّا لِإِمَامٍ أَوْ نَائِبِهِ
It is forbidden to establish a legal punishment unless it is done by the leader or his deputy.
Source: al-Furū’ wa Taṣḥīḥ al-Furū’ 10/29
And according to the Kuwaiti Encyclopedia of Fiqh:
اتَّفَقَ الْفُقَهَاءُ عَلَى أَنَّهُ لاَ يُقِيمُ الْحَدَّ إلا الإمام أَوْ نَائِبُهُ وَذَلِكَ لِمَصْلَحَةِ الْعِبَادِ وَهِيَ صِيَانَةُ أَنْفُسِهِمْ وَأَمْوَالِهِمْ وَأَعْرَاضِهِمْ
The jurists came to a consensus that a legal punishment may not be implemented unless by the leader or his deputy. That is in the best interest of people, which is to safeguard their lives, their property, and their reputations.
Source: al-Mawsū’at al-Fiqhīyah al-Kuwaytīyah 17/144
Therefore, a Muslim who witnesses a crime should inform the local authorities who can take appropriate action. Muslims do not have the right to commit acts of vigilante justice or to take the law into their own hands without due process.
Success comes from Allah, and Allah knows best.