Origin of the "Fear and Loathing" phrase

I'm reading Proud Highway now and was moved to learn that the first known instance of the phrase "Fear and Loathing" was in a letter HST wrote to William Kennedy on 11/22/63. He summarized his feelings at that moment as being full of fear (for the country) and loathing (for the people who did it). I always wondered where that came from, and now I can see an old editor like William Kennedy telling HST that that was a great line.