Cockburn attacks Hitchens in the letter for his "increasing seclusion in fantasy". He speculates that Hitchens may be suffering from delusions like other advanced drinkers. He also attacks Hitchens for trying to get Sid Blumenthal "nailed by Congress on a perjury rap". It was this latter case which prompted the Judas reference.
As best I can tell, Hitchens saw Bill Clinton and the risk he posed to our nation, more clearly, regardless of state of inebriation, than any of our U.S. Senators, including the Republicans who allowed the trial to be just a charade.