[The Counsel of the Commander of the Faithful, Peace be upon Him, Regarding Fighting the Khārijites]

The answer: By this counsel, the Commander of the Faithful, peace be upon Him, intended to forbid his son al-Ḥasan, peace be upon Him, from fighting the Khārijites under the banner of Muʿāwiyah and under his command. For Muʿāwiyah and his companions—even though they fought the dogs of the people of the Fire—were more misguided and more unjust than the Khārijites. That is because the Khārijites did not enter into misguidance deliberately; rather, they entered it through error in judgment, so their intent was the truth but they missed it. As for Muʿāwiyah and his companions, they entered into misguidance deliberately and out of love for misguidance.
And it is not the intent of the Commander of the Faithful Imām ʿAlī (Peace be Upon Him) that fighting the Khārijites is absolutely impermissible. Rather, his intent is what we have mentioned, namely supporting Muʿāwiyah against the Khārijites.
And the Commander of the Faithful (Peace be Upon Him) clarified the reason for his forbidding his son al-Ḥasan (Peace be Upon Him) when he said: “For the one who sought the truth…” etc. The meaning of that is: because Muʿāwiyah was more misguided than the Khārijites and more deeply immersed in falsehood than they were.
Then the Commander of the Faithful (Peace be Upon Him) clarified that Muʿāwiyah went astray from two aspects: choosing falsehood deliberately, with intent, pursuit, and resolve—this is one aspect; and the second aspect is his attaining falsehood, entering into it, and acting upon it.
As for the Khārijites, they only went astray from one aspect, in that they had no intention of falsehood, nor pursuit, nor resolve, nor determination. Their intention was only to seek the truth, but they erred in it, and thus entered into falsehood and acted upon it; so their misguidance was only from this one aspect.
If it is said: Was it then permissible to fight Muʿāwiyah together with the Khārijites?
We say: There are texts indicating that it is not permissible to assist the people of falsehood in their falsehood, such as the saying of Allah the Exalted: “And cooperate in righteousness and piety, and do not cooperate in sin and transgression” [Al-Māʾidah:2]. And the Khārijites are people of misguidance; they are the worst of creation and creatures. They pass out of the religion as an arrow passes out of the game. Therefore, assisting them is not permissible.
If it is said: If Muʿāwiyah wronged them by aggression and transgression, and their stance was merely one of defense, then why is it not permissible to assist them in repelling injustice from them, just as it is permissible and obligatory to repel injustice from the People of the Covenant (ahl al-dhimmah)?
We say: Even though the Khārijites may be as such, assisting them is not permissible, because they have, through their misguidance, departed from the guardianship of Allah to enmity toward Him, and they have departed from the party of Allah to the party of Satan. And Allah the Exalted described the believers in Sūrat al-Tawbah as being allies of one another, and that the hypocrites are of one another.
And He, Glorified and Exalted, said: “And thus We appoint some of the wrongdoers over others because of what they used to earn” [Al-Anʿām:129]. In this verse there is evidence that what befalls some wrongdoers from others is a type of immediate punishment which they deserved because of what they used to earn. Thus, it is neither proper nor permissible for the believer to confront the wrath and punishment of Allah that descends upon the wrongdoers.
What is befitting of the believer—and more appropriate for him—is to keep away from that and to say: “Our Lord, do not make us a trial for the wrongdoing people” [Yūnus], and: “And do not place us with the wrongdoing people” [Al-Aʿrāf], and: “So separate between us and the defiantly disobedient people” [Al-Māʾidah].
If it is said: The people of the school have said that it is permissible to assist the lesser wrongdoer against the greater wrongdoer.
We say: They said that with conditions:
1- That their assistance not be in matters exclusive to the Imams.
2- That the wrongdoer adhere to sound judgment, meaning in accordance with the dictates of the Sharīʿah.
3- That assisting him not lead to strengthening his oppression.
As for the People of the Covenant, defending them is obligatory due to what the covenant of dhimmah entails: the protection of their lives, their children, their women, and their wealth—on the condition that they pay the jizyah willingly while being subdued.
As for the Khārijites and the like, by their departure from guardianship the bonds have been severed between them and others. They have no share or portion in the guardianship of the believers, nor do they possess, like the People of the Covenant, a treaty or covenant.
If it is said: If the aggressing enemy against the Khārijites is from the disbelievers, does the ruling differ?
We say: The ruling here differs from what preceded. Assisting the Khārijites and the like is permissible in repelling the aggression of the disbelievers, because the advance of the disbelievers against the Khārijites or the like is considered an advance against Islam. Hence Allah the Exalted said in what resembles this: “And those who believed but did not emigrate—you have no guardianship over them at all until they emigrate. But if they seek your help in the matter of religion, then upon you is the help…” [Al-Anfāl:72]. So when the disbelievers are repelled from the lands of the Muslims, the Muslims then withhold their hands from assisting the Khārijites.
If it is said: What is meant by the saying of Allah the Exalted: “And if two groups of the believers fight, then make peace between them. But if one of them transgresses against the other, then fight the one that transgresses until it returns to the command of Allah…” [Al-Ḥujurāt:9]?
We say: What is meant by this verse is that if two groups of Muslims, or two tribes, fight one another, then it is obligatory upon the believers to reconcile between them. If one of them then transgresses after reconciliation against the other, it becomes obligatory to fight the transgressing party until it ceases its transgression and returns from its enmity.
As for the Khārijites and Muʿāwiyah, they are certainly outside the command of Allah, and before Allah they are transgressing wrongdoers. Their matter is not ambiguous. What would have been obligatory upon the believers—had the means been available to them— would have been to fight both parties, not to reconcile between them. And likewise is the obligation concerning anyone who is of their description.
Yes, if a Khārijite repents, and he had killed soldiers from among the soldiers of Muʿāwiyah or the like, then what appears is that nothing is due from him except sincere repentance.
1- Because the Khārijites are among the disbelievers, even though their disbelief is disbelief by interpretation. And it is known that Islam erases what came before it, and repentance from disbelief erases what preceded it.
2- Because the Commander of the Faithful Imām ʿAlī (Peace be Upon Him) did not pursue the people of Basra after the Battle of al-Jamal, and it is known that they had treacherously killed a number of his companions before the battle, and killed others during the battle. Yet the Commander of the Faithful (Peace be Upon Him) did not question them regarding the killers.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3