Imam Abu Abdullah Al-Qurtubi (d.671H) in his book al-Jami li Ahkam al-Quran under explanation of verse 2:179 states:
It is agreed upon by the Imams who give fatwa that it is not permissible for anyone to take his right from anyone without the authority/sultan. It is not (permissible) for the people to take revenge on one another, rather that is for the Sultan or whoever is appointed by the Sultan for that. That is why God has set the (requirement for a) Sultan in order to hold off peoples hands from each other.
Despite becoming aware of the above related ijma on this matter, if one yet ideologically persists in such vigilantism than this could perhaps be due to the following reasons;
His delusion that his faith, his station and his depth of understanding of the inner reality of the rulings of the Sharia is at higher level than the common folk and that of the Imams and this reality is only known to Allah. As such he deludes himself to be not bound by the Sharia. This is similar to claims of the heretical Batiniyya sect.
His rejection of the authority of the Imams and his delusion of being a mujtahid who derive rulings directly from the Quran and Sunnah. Hence when based on his ignorance and desires and having no authority for ijtihad nor having any usul nor mastered any of the books of fiqh let alone the madhab itself, he comes to an understanding of the Quran and Sunnah that contradicts the understanding of the Imams and their Ijma, than he rushes to “throw away” the ruling of the Imams against the wall and gives preference to his personal opinion. Or he takes rulings from individuals who has these characteristics. And this is similar to the case our Prophet عَلَيْهِ ٱلصَّلَاةُ وَٱلسَّلَامُ warned against in the report below:
Abdullah b. ‘Amr b. al-‘As reported Allah’s Messenger (may peace be upon him) as saying: Verily, Allah does not take away knowledge by snatching it from the people but He takes away knowledge by taking away the scholars, so that when He leaves no learned person, people turn to ignorant as their, leaders; then they are asked to deliver religious verdicts and they deliver them without knowledge, they go astray, and lead others astray. (Sahih Muslim)
Wahhabism Unveiled 