What Does the Prophet's Description of the Khawarij — Abundant Prayer and Worship — Indicate?

Question

The Messenger (peace and blessings be upon him) described the Khawarij with specific characteristics, among them abundant prayer and worship. What does this indicate? Is it for the sake of worldly gain, or what?

Answer (English translation)

His describing them with worship and recitation of the Qur'an and the like indicates that they have — and we seek refuge in Allah — an excessive strictness in religion, but this religion has not reached the hearts. This is found in those who busy themselves with the faults of people while neglecting their own faults; you find his heart empty of connection with Allah the Exalted, merely circling around the world: what happened? You find in his heart a burning zeal — but if you were to examine him regarding turning repentantly to Allah the Exalted and connecting with Him, you would find his heart empty. This is the reason.

For this reason, we forbid ourselves before anything else from letting our concern be nothing but preoccupation with the affairs of people which, if they are wrong, we cannot set right. And you find the likes of these people seeing the truth as falsehood, or seeing the truth as truth but buried beneath far greater falsehood — meaning they count the things that deserve criticism and abandon the things that deserve praise. So they go astray and lead others astray, and this is a very dangerous matter. There is no need for me to give you examples of what evil and tribulation some Arab countries have suffered because they approached the matter in the wrong way.