AUTHOR'S NOTE
"Anybody who develops an interpretation such a considering the killing of innocent people to be a noble act which will take him to paradise, or doing takfir on Muslims; is an act of disbelief. There is no Islamic funeral (Janaza) for disbelievers."
Ibn Taymiyyah makes the point that in this particular section he admits there are two opinions on the matter and that the first is that "they are disbelievers like apostates". (Majmu'a al-Fatawa 28:518)
It is incorrect for people to make the assumption that due to Ibn Taymiyyah quoting a historic event in his works therefore he was also of this view. The event is as follows:
"The Khawarij who deviated, whom the Prophet (blessings and peace of Allah be upon him) enjoined us to fight, and whom Ameer al-Mu’mineen ‘Ali (may Allah be pleased with him), one of the Rightly-Guided Caliphs fought, and whom the leading scholars of Islam among the Sahabah, Tabi‘een and those who came after them were unanimously agreed upon fighting, were not described as disbelievers by ‘Ali ibn Abi Taalib, Sa‘d ibn Abi Waqas and others among the Sahabah; rather they regarded them as Muslims even though they fought them, and ‘Ali did not fight them until they shed blood unlawfully and raided the property of the Muslims; then he fought them in order to ward off their wrongdoing and aggression, not because they were disbelievers. Hence he did not take their womenfolk captive and he did not seize their wealth as booty." (Majmu'a al-Fatawa 3:282)
However his own actual position is in his work al-Nubuwwat and derived from when he quoted the words of the Holy Prophet (peace & blessings be upon him) himself: “’They shall exit from Islam just as an arrow exits a hunted game’. [Ibn Taymiyyah opined:] Their exiting from it means their leaving it due to their belief that it is lawful to shed the blood of Muslims and seize their wealth”. (pg. 225)
This is an opinion of a "large body of scholars" (Asqalani 12:313).