So accepting for a moment that these draconian instructions were written in times when tribal leaders had fewer options available to them with respect to managing miscreants and maintaining some sort of law and order, I suspect that most people today would agree that killing people is a bad thing and should not be condoned except under extraordinary circumstances.
My friend and I then proceeded to try to list those circumstances. We started with self-defence or perhaps protecting a vulnerable person like a child from being hurt. Fair enough? But what if you had the capability instead of killing the source of the danger, to disable him or her and then let the authorities take over for imprisonment and rehabilitation? Are there any circumstances where killing would still be preferable if a non-lethal option were available and the danger permanently removed to your satisfaction? And might there be circumstances where killing someone, no matter how justifiable you felt their eradication might be, where in doing so you create a Gorgon-like multi-headed beast, far worse than the one you removed.
Which brought us on to the leaders of IS and Boko Haram, the current epitomes of evil. We both eagerly agreed on that. Presumably they're the Amaleks of our age. If I had control of a drone which with the press of a button I could accurately kill these deranged and evil people, painlessly (ideally, although perhaps not popularly) and with no danger of collateral harm to anyone else, would I press the button? I think most people on this planet right now would willingly press it. It won't solve the problem of Islamic fanatics, but hopefully we'd be slowing them down a touch. We'd certainly feel better living in a world without these demented leaders. Of course we would probably only have hardened the resolve of their acolytes and assisted their recruitment drive - especially if our assassination included collateral deaths.
But if we applied the same alternative to our self-defence conundrum, namely providing a second button on that drone which only stuns our evil crazed targets, would we be able to suppress our anger and desire for revenge? Would we press the non-lethal button, hypothetically assuming that something like British police and justice would be around to subsequently imprison and rehabilitate the bastards etc etc? Could we control our rage or would we be tempted to incisively, easily, safely and painlessly remove them from society forever?
Clearly this is hypothetical - although drones might indeed be deployed to try to take them out. And if they do, let's hope they can identify and target accurately. But even then, who orders it and under whose authority? Who will be held to account for killing another human, and one who was probably not within a territory over which they had been elected to make those sorts of decisions.
But the moral question remains. Are there any circumstances where killing anyone for any reason is justified when disabling them is available as a practical alternative? Can we suppress our natural urges for reprisal and revenge? Indeed should we? Does killing a bad person send appropriate signals to others who might be tempted to step into their shoes - although jihadists will probably feel more compelled to avenge their leader's martyrdom. So killing them probably increases their numbers and resolve, not diminish it. Witness the rise of IS and the strengthening of the Taliban following many years of their families enduring carnage and deprivation as well as being inflicted with the 'wrong sort of Islam'. No wonder they want to fight back, although no-one in their right mind would ever think of deliberately burning someone to death in a cage and proudly filming it is the work of a sane person.
So can we become Better Angels of our Nature, as Steven Pinker brilliantly puts in his book about the decline of violence, and honestly state we'd only ever press the non-lethal button if we could be sure the danger will be removed without loss of life? Does our willingness to press the kill button move us uneasily towards the domain of the creators of the Final Solution, Boko Haram and IS? I have no doubt that every member of these dreadful organisations also believed, no matter how distorted their logic, that they were doing good by removing people who were a danger to them in some way.
We must always search for better ways to deal with adversity than resorting to bestial reactions. We must keep evolving. But it takes effort and resolve. Listen to a politician I admire, Rory Stewart, explaining why we shouldn't have been in Afghanistan.