Shaikhul Islaam Ibn Taymiyyah (rahimahullaah) said:

”The leaders of the people of Bidah are more harmful upon the Ummah than the sinful people; so  because of this the Prophet (sallal-laahu-alayhi-wasallam) commanded that the khawaarij are killed (i.e. fought and killed by the Muslim ruler) and he prohibited (us) from fighting the oppressive rulers.” [Majoo Al-Fataawaa 7/284]