Then their evil increased during the Khilaafah of ‘Alee (radyAllaahu ‘anhu) and they rebelled against him, declaring him to be a disbeliever. They also pronounced disbelief on the Companions, because they would not agree with them in their (false) beliefs. So they ruled that all those who opposed them in their views were disbelievers. As a result, they pronounced disbelief on the best amongst creation – the Companions of Allaah’s Messenger (sallAllaahu ‘alayhi wa sallam). Why was this (?) – because they did not agree with them on their misguidance and disbelief.
Their Beliefs: They do not adhere to the Sunnah and the Jamaa’ah, nor do they obey the leader. Rather, they hold rebelling against him and renouncing allegiance to him to be from the Religion, contrary to the advice of Allaah’s Messenger of giving obedience and contrary to what Allaah has commanded in His saying: “Obey Allaah and obey the Messenger and those in authority amongst you.” [Surah An-Nisaa: 59]
So Allaah made obeying the ruler part of the Religion, and the Messenger of Allaah (sallAllaahu ‘alayhi wa sallam) made obeying the ruler part of the Religion, as is found in his saying: “I counsel you to have Taqwaa of Allaah and to hear and obey, even if a slave should take command over you. For indeed, whosoever lives amongst you, will see many differences…” 
So obeying the Muslim Ruler is part of the Religion (of Islaam). But the Khawarij say: “No, we are free (from this).” This is the way of insurrection and rebellion today!
So the Khawaarij are those who seek to cause division to the main unified body of Muslims and to revolt against the leaders – and there is disobedience to Allaah and His Messenger in doing this. They also hold that the person who commits a major sin is a disbeliever.
So someone that commits a major sin – a fornicator, a thief, one who drinks alcohol, for example – are all considered disbelievers by them. On the contrary, Ahl-us-Sunnah wal-Jamaa’ah hold such a person as a “Muslim with deficient Eemaan” and they call him a “sinner in the Religion.” So he is a “believer” due to his Eemaan, while being a “sinner” due to his major sin. This is because nothing expels one out of the fold of Islaam except Shirk and the well-known things that nullify one’s Islaam. As for the sins that are below Shirk, then they do not cause one to be expelled from the fold of Eemaan, even if they are major sins. Allaah says: “Verily Allaah does not forgive that Shirk (association of partners in worship) be committed with Him, but he forgives what is lower than that (of sins) to whom He wills.” [Surah an-Nisaa: 48, 116]
The Khawaarij say: “The one who commits a major sin is a disbeliever, he will not be forgiven and he will reside in the Hellfire forever.” And this is contrary to what is stated in the Qur’aan. The reason for this is because they do not have understanding (of the Religion).
Take note that the cause for their falling into these (false beliefs) was their lack of knowledge. This is because they are a people intense in their worship, prayer, fasting and recitation of the Qur’aan. And they have a strong fervor for the Religion, but they do not have knowledge – and this is the problem.
So going to great lengths and exerting oneself in piety and worship must be accompanied by knowledge and understanding of the Religion. This is why the Prophet (sallAllaahu ‘alayhi wa sallam) described them to his Companions in the following manner: That the Companions would look down upon their own prayer as compared to their prayer and on their worship as compared to their worship. Then he said: “They will shoot out from the Religion just as the arrow shoots out from a hunted game” – in spite of their worship and in spite of their righteousness and their praying Tahajjud at night.
So because their exertion was not based upon a correct foundation, nor upon authentic knowledge, it became a misguidance, a plague and evil upon them and upon the ummah.
And furthermore, it is not known from the Khawaarij – for one day – that they fought against the disbelievers ever! Instead, they only fight with the Muslims, as the Prophet (sallAllaahu ‘alayhi wa sallam) said: “And they will kill the People of Islaam, while leaving alone the people of Awthaan (idols).” 
So we have not come to know in the history of the Khawaarij that one day they fought against the disbelievers and polytheists. Rather, they are always fighting against the Muslims. They killed ‘Uthmaan and they killed ‘Alee bin Abee Taalib. They killed Az-Zubayr Ibn-ul-‘Awaam and killed the best of the Companions. And they have not stopped killing Muslims.
And this is all due to their ignorance of the Religion of Allaah. But in spite of this, they had piety and worship, but since these were not founded upon authentic knowledge, it became a disease on them. This is why the great scholar, Ibn Al-Qayyim (rahimahullaah) said in his description of them:
“They have textual evidences, which they fall short of in understanding So they have been given shortness in knowledge.” 
So they use texts as evidences but yet they do not comprehend them. They use as evidence texts from the Qur’aan and the Sunnah concerning the threat for committing sins, but they do not comprehend their meanings. They do not refer them back to the other texts, in which there is found a promise for forgiveness and the acceptance of repentance for those whose sins are less than Shirk. So they accept one part and leave off another part – all of this due to their ignorance.
So having an over-protective love for the Religion and enthusiasm are not sufficient. They must be founded upon knowledge and understanding of Allaah’s Religion. This is so that they can be produced from knowledge and so that they can be put in their proper place.
So over-protective love for the Religion is good and enthusiasm is good, but they must be guided and directed by the following of the Qur’aan and the Sunnah.
There is no one with more precedence in the Religion nor more sincere to the Muslims than the Sahaabah (the Comapnions), but in spite of that, they fought against the Khawaarij due to their danger and their evil.
‘Alee bin Abee Taalib fought against them such that he slaughtered them with the worst of killings in the incident of Nahrawaan. By doing this, he realized what the Prophet (sallAllaahu ‘alayhi wa sallam) informed us of in that the Prophet (sallAllaahu ‘alayhi wa sallam) gave the good tidings to the one who killed them of goodness and Paradise. So ‘Alee bin Abee Taalib, he was the one who killed them – so he received this good tiding from Allaah’s Messenger. He killed them in order to prevent their evil from befalling the Muslims.
It is an obligation on the Muslims in every generation, if they should become aware of the existence of this wicked methodology, that they remedy it by calling to the Way of Allaah first and educating the people about it. But if they do not accept this, then they must fight against them in order to prevent their evil.
‘Alee bin Abee Taalib (radyAllaahu ‘anhu) sent his cousin, ‘Abdullaah bin ‘Abbaas – the Habr (scholar) of the ummah and the Turjumaan (Interpreter) of the Qur’aan – to them. So he debated with them and six thousand amongst them repented and returned back, but many remained behind and did not repent. So at this point, the Ameer-ul-Mu’mineen, ‘Alee bin Abee Taalib, along with the Sahaabah, fought against them. This was in order to prevent their evil and harm from befalling the Muslims.
So this is the sect known as Al-Khawaarij and their beliefs.
[By: Shaikh Saalih Al-Fawzaan|Lamha ‘an-il-Firaq-id-Daalah (pg. 31-37) w/ notes by Shabbaab Ar-Raajihee|Courtesy of: Al-Ibaanah.com]