"You'll be better off if you spare my life"

I'm currently involved in a negotiation in which the other party is as nasty and bloodthirsty as the guy on the right.  Their poor grasp of reality has led them to imagine I've wronged them in ways that never happened.  They feel justified now in attacking me viciously.  What few facts they DO understand, they distort without hesitation or shame in their zeal to do me harm.

So what can I do?  How can I negotiate effectively to get an outcome that meets my needs?  I can't appeal to their sense of fairness, because they are not fair.  I can't appeal to their conscience because it is missing.  I can't reason with them because they are irrational.

All that is left is appealing to their interests.  Make it clear that this attack is not in their interest.

This attack, as nasty as it has been, at least has reinforced everything I've read about negotiation.  I can see why the classic book, Getting to Yes: Negotiating Agreement Without Giving In, describes the core of successful negotiation as identifying and then addressing both your own interests and the interests of the other person.  

Fortunately, 99% of the time, I negotiate with decent people. Both parties want what's good and fair, but have different priorities and perspectives.  But in the worst case, when I'm dealing with a really nasty person, I've learned that I can still broker a deal.  The other person may hate me, may want to do me harm, may be dishonest, unreasonable, deluded, and corrupt--but they have interests.  I must find those interests.  And make them see that those interests will best be served if they change their position.