To know the Khawarij of today, one needs to know well the Khawarij of the past. The following book provides a good detailed look in English.
Note that the author writes from the perspective and language of an orientalist and so a little bit of his own naturalist/skeptic theorizing will be present.
An example of how the past characteristics help understanding them, is for example the statement found in the hadith regarding the Khawarij that they would “leave the non-Muslims and kill the Muslims”. Now although we have shown before how this applies to Wahhabism yet there are those of today who attempt to confuse people by showing that these terrorist Wahhabis of today don’t leave the non-Muslims alone and instead it is the secularist/non-practicing Muslims who align with the Non-Muslims and hence it is not they but these secular/non-practicing Muslims who are Khawarij. Such extreme distorted defense mechanism is common among them. To respond to this, one just needs to look at the history of these Kharijites and one would realize that they were the farthest away from being classified as “secularist”. They were on the contrary the most hardcore in their insistence on conformity to the Quran and Sunna. And this characteristic of leaving alone the non-Muslim while killing the Muslims, was a result of their takfiri doctrine that led them to use most of their energy and efforts and brutality against Muslims whom they considered as apostates, while in case of the Non-Muslim they showed them far more tolerance by considering them as “dhimma” who needs to be protected and to whom the message of Islam should be taught. This does not mean the Khawarij totally left alone the non-Muslims or that they did not apply hard laws against them or that they abandoned jihad. Most of their laws regarding dhimmis matched with the Sunnis. The difference instead was that because they made takfir on Muslims they took a harder position against Muslims and hatred for them. And this is exactly we find with many of the Kharijites of today that their hatred and treatment of Muslims is worse than they would treat the Non-Muslims, and their fighting and killing of Muslims is more than that with Non-Muslims.
In a similar way many other characteristics of the Khawarij can be understood by a careful reading of the past. Another example, is that we notice the Khawarij of the past were interested in hadith studies and took a lot of effort in transmitting and studying them such that even among Sunni scholarship, there were Khawarij narrators who were considered as reliable. One should understand from this that just because many of the Wahhabis of today with their “Quran and Sunnah” slogan try hard to master hadith, it does not in any away prove their closeness to Sunni Islam. Another aspect also to note from the Khawarij of the past, was that they were roughly of two types, one being the pacifist/ isolationist/preaching type and one being militant. The division can also be seen as one who are engrossed in theological debates and writing and studies, while the other being ignorant Bedouin types who rally and do their dirty work for them. Fast forward to today, and we see two types of Wahhabis, the militant and the preaching type. This division in fact existed even during the earlier periods of Wahhabism, which ended with the preaching type who aligned with the House of Saud crushing the more militant types who were called “Ikhwan”. But that didn’t end it fully, and the division have reemerged again in our time, with self declared “jihadi” type and the house of Saud aligned preaching type.