I couldn’t understand what the reasoning was behind killing Abdullah ibn Khabbab by the Khawaarij. Was it because he narrated a hadeeth that was opposed to their actions?

The Khawaarij used to interrogate every person who they met and find out his position towards their concerns before letting him pass. If the person did not share their belief that all those who participated in the arbitration had apostatized, they would kill him – without any Islamically justifiable reason. This is what is meant by their principle of isti‘raad.

In this case, they had all the more reasons for killing Ibn Khabbaab as he was appointed a governor by ‘Ali (radiyallaahu anhu) over one of the neighbouring regions. (C.f. Fath-ul-Baaree, 12/284).

For details about the incident, see:

• Ibn al-Jawzee’s The Devil’s Deception (pp.32-33) by Dr. Bilal Philips
• Ali ibn Abi Talib (2/274-275), by Dr. Ali M. Sallabi

And Allaah knows best.