Imaam Ibnul Qayyim (rahimahullaah) said:
When he (radiyallaahu-anhu) knew that he would be killed due to what he could see and observe regarding the events and signs, he refrained from fighting and from defending himself lest fighting occurs between the Muslims, whose final end result would lead to his murder, thus he preferred to be killed instead of (starting) a fight between the Muslims.
[الطُّرُقُ الْحُكميَّة- page 30
NB: This does not mean that the authorities do not fight the khawaarij when they engage in their satanic crimes. See article here http://salaficentre.com/2014/07/imaam-barbahaaree-speaks-about-the-khawaarij-terrorists/