Allaamah Rabee Bin Haadi Al-Madkhalee [may Allaah preserve him] said:

By Allaah the Ummah will not exit from its problems, the humiliation and degradation they are living in, except by way of returning to the Book of Allaah, the Sunnah of the Messenger [sallal-laahu-alayhi-wasallaam] and the understanding of the Salafus Saalih in [dealing with] the affairs of the khawaarij and others. Imaam Al-Aajurree [rahimahullaah] said: “The scholars do not differ–past and present–that the khawaarij are an evil people, disobedient to Allaah and His Messenger, even if they [perform] the prayer, fast and strive in [carrying out] acts of worship.

[Source: ( الذريعة إلى بيان مقاصد كتاب الشريعة للإمام المحدث أبي بكر محمد بن الحسين الآجري   page: 97-98. Vol 1). slightly paraphrased]