Khawarij were explained by Muhammad

……and he did not like them, but he did make a revelation…….”… Blessed are those who fight them and are killed by them. They call to the Book of Allah but they have nothing to do with it. Whoever fights them is better to Allah than them.”

The Messenger of Allah said: At the end of time (meaning: the last days of the world), there will appear some people among you who are young in age, immature and senseless; they will use the best of the speech of the people in their claims, and they will leave Islam just as the arrow pierces the body of the game and then abandons it. Their faith will not go past their throats, so wherever you see them, kill them; for their killing will be recompensed and rewarded on the Day of Judgement. (Bukhari)
This is evident in the members of these organizations—in what they preach and how they defend themselves. In fact, those who have left these groups after having been members confess that their judgement had been clouded.
As is pointed out in the aforementioned hadith, they use the Quran, the Sunnah, and scholarly statements to support their claims, despite the fact that all of these evidences and proofs are against them and not for them.
The Prophet said: They recite the Quran but it does not go past their throats, meaning that the teachings of the Quran do not enter their hearts, nor do they understand or comprehend them.
The Messenger of Allah said: Your prayer does not compare to their prayer, nor does your fasting compare to their fasting, nor does your recitation compare to their recitation (Muslim).
The Messenger of Allah said: They kill the Muslims and spare the idolaters. If I live to see them I will kill them like the killing of (the people of ‘Aad). (Bukhari and Muslim).
The Prophet said about the ideological progenitor of the Khawarij, Dhul Khuwaisirah: Leave him, for there will issue from his loins (progeny) a people who will go to the extreme in religion until they abandon it altogether, just as the arrow pierces the body of the game and then abandons it. (Ahmad)
In this, they tread in the footsteps of their ideological ancestor, Dhu Al-Khuwaisirah, who said to the Messenger of Allah as he was distributing the war booty: O Muhammad, be just [in your distribution of the booty], for this is not the distribution of a man who seeks Allah’s pleasure. (Ibn Majah) For this reason, the scholars have considered Dhul Khuwaysarah to be the progenitor of the Khawarij.
They did not appear in Syria, Iraq, and other war-torn countries except when there was disunity and discord in these lands. Similarly, the earlier Khawarij movement intensified after the murder of caliph ‘Uthman ibn ‘Affan, an incident that stirred up disruption and disagreement among the Prophet’s Companions.
They consider themselves more knowledgeable than ‘Ali ibn Abi Talib, Ibn ‘Abbas, and the rest of the Companions. Consequently, they have caused the Companions a great deal of trouble and harm.
Ibn ‘Umar said: “They (the Khawarij) took verses about the disbelievers and used them against the believers.” (Bukhari)
Al-Safirini said concerning the opinions of Nafi’ ibn ‘Abdullah Al-Azraq (the ideological founder of Al Azâriqa sect of the Khawarij), “He accuses those who disagree with him of apostasy and considers it legal (halal) to kill them.” (Lawami’ Al-Anwar, 1/86)
Despite the abundance of ahadith recommending patient endurance with tyrannical leaders [if carrying arms against them will be counterproductive. And this of course does not mean neglecting the duty of al–amr bi al-ma‘ruf wa al-nahi ‘an al-munkar towards these rulers in a wise and peaceful manner.]The Khawarij advocate the obligation of assassinating such leaders and all those who are pleased with their rule or support and help them.
Their sects include the Azâriqa, the Ibadhiyah, Al-Najdât and several others. That’s how the Khawarij are seen today: killing and accusing each other of apostasy.
Imam Al-Ajurri said: “They emerged from various regions and united under the banner of Promoting Virtue and Preventing Vice until they reached Madinah and assassinated Caliph ‘Uthman ibn ‘Affan.” Don’t be Misled by Today’s Khawarij
There would arise at the end of the age a people who would be young in age and immature in thought, but they would talk (in such a manner) as if their words are the best among the creatures. They would recite the Qur'an, but it would not go beyond their throats, and they would pass through the religion as an arrow goes through the prey. So when you meet them, kill them, for in their killing you would get a reward with Allah on the Day of Judgement. Sahih Muslim - Book 12 Hadith 199 The Book of Zakat.