Difficult student would dispute every grade in the course and now wants letter of recommendation
It sounds like you've agreed to write a letter so I think you are committed unless the student releases you from your commitment.  (Personally, I'd have said no in the first place and explained if asked that, based on their behavior, I could not honestly give a favorable recommendation.  But that ship has sailed.)

But what you haven't agreed to yet, I hope, is a favorable letter.  What I would do in your situation is contact the student to explain that on reflection, you've become concerned that the only honest letter you can write will not be helpful.  You don't feel you can honestly ignore that the student is a difficult, argumentative, confrontational and disrespectful individual.  To be an honest appraisal, that has to go in there.  You cannot write an honest letter that only recites the good stuff.

At that point, you can ask, do they still want the letter?  I'm sure they'll be difficult and disrespectful once again but that's nothing new and if you stand your ground, I think they'll decide to go elsewhere.  Problem solved.  (And maybe they'll learn something from the experience, though, from what you describe, probably not.)