We were in a small house in Zarqa, Jordan, trying to interview two heavily bearded Islamic militants about their distribution of recruitment videos when one of us asked one too many questions.

"He's American?" one of the militants growled. "Let's kidnap and kill him."

The room fell silent. But before anyone could act on this impulse, the rules of jihadi etiquette kicked in. You can't just slaughter a visitor, militants are taught by sympathetic Islamic scholars. You need permission from whoever arranges the meeting. And in this case, the arranger who helped us to meet this pair declined to sign off.

"He's my guest," Marwan Shehadeh, a Jordanian researcher, told the militants.

With Islamist violence brewing in various parts of the world, the set of rules to guide and justify the killing that militants do is growing more complex...

...Some of these rules have deep roots in the Middle East, where, for example, the Egyptian Islamic scholar Yusuf al-Qaradawi has argued it is fine to kill Israeli citizens because their compulsory military service means they are not truly civilians...

Rule No. 1: You can kill bystanders without feeling a lot of guilt.

The Koran, as translated by the University of Southern California Muslim Student Association's Compendium of Muslim Texts, generally prohibits the slaying of innocents, as in Verse 33 in Chapter 17 (Isra', The Night Journey, Children of Israel): "Nor take life, which Allah has made sacred, except for just cause."

But the Koran also orders Muslims to resist oppression, as Verses 190 and 191 of Chapter 2 (The Cow) instruct: "Fight in the cause of Allah with those who fight with you, but do not transgress limits; for Allah loveth not transgressors. And slay them wherever ye catch them, and turn them out from where they have turned you out, for tumult and oppression are worse than slaughter."

In the typical car bombing, some Islamists say, God will identify those who deserved to die - for example, anyone helping the enemy - and send them to hell. The other victims will go to paradise.

Rule No. 2: You can kill children, too, without needing to feel distress...