As for the statement that the Khawaarij are people of 'ibaadah and worship, then this is true. The Messenger, sallallaahu 'alayhi wa sallam, has indeed described them by stating that, "There will come a people, and you will render as insignificant your salaah compared with their salaah, and your fast compared with their fast. They will recite the Qur'aan but it will not pass their throats."
So where is the deviance? The source of deviance lies in two matters: it is either because of corrupted intentions which hinder the person from righteous deeds, or it can be because of having a corrupted understanding. A person can worship and make great effort; however his understanding of Allaah's deen is incorrect.
So just as having shortcomings in one's obedience is a sin, likewise making additions to that which is obligatory is a sin. Thus, the deviance of the Khawaarij who made takfeer of 'Alee and those who sided with him was due to their corrupted understanding. And having a corrupted understanding is more dangerous than having a corrupted intention. Hence, the one who understands a matter of the deen incorrectly and therefore transgresses against the deen, in spite of having a good intention, is more dangerous to Islaam and the Muslims than the one whose understands Islaam correctly but his intentions are wrong. The person who commits a major sin such as fornication or drinking would admit to his wrongdoing. However, how would the innovator repent whilst he thinks that he is worshipping Allaah by the bid'ah he commits? The Khawaarij are people of desires, they fight the rulers for authority, and this is why they made takfeer of the people.
I came a across and athar from Al-Hasan Al-Basree that was mentioned by Ibn Hiyyaan At-Tawheedee in his book 'Al-Basaa'ir wad-Dakhaa'ir' in the first volume on page 156 wherein he gives insight into the deviance of the Khawaarij. He said:
A man came to Al-Hasan Al-Basree and asked him what does he think about the Khawaarij. So he replied, "Hum as-haabu dunyaa" (i.e. They are a people who are chasing after this world). So he asked him, "How is it that you say that they are people of this world whereas one of them would plunge himself in the midst of shooters until he is dead, and he would leave behind his wife and children."
So Al-Hasan said, "Tell me about the ruler! Do they prevent you from establishing the salaah, giving the zakaah, performing the Hajj and 'Umrah? What I see is that they withhold from you the possessions of this world and that is why you fight them." End of his statement.
That is, you fight them so that you can be the ruler and you can be the one in charge. Thus, according to Al-Hasan Al-Basree, the Khawaarij are people of this world and their deviance is in their understanding of the deen.
Shaykh Mashhoor Hasan Salmaan
 Al-Bukhaaree (hadeeth no. 3414)