Shaikh Saaleh Al-Fawzaan [may Allaah preserve him] says:

“The people of knowledge [i.e. the scholars] are of the view that it is obligated on the Muslims in every era that if they are certain about the presence of this filthy doctrine [i.e. the doctrine of the khawaarij], they should remedy it through calling to Allaah [i.e. the correct way] and enlighten the people about that. If they [i.e. khawaarij] do not obey [and come out fighting], then they are fought [by the Muslim ruler] in order to repel their harm.”

[لمحة عن الفرق الضالة page 42]