not revenge or hate but possibly he does not want his cooperation revealed. He may have compromised secrets that put many other Americans at risk. He may have even turned to endorse the communists and denounce the war.
the Vietnam war (police action) was obviously lost due to interference within our own government. There were 3 sides; the US military that wanted to win, the US government socialists that wanted us to lose, the US government and citizens that thought it was none of our business and we shouldn’t have even been involved.
Perhaps releasing too many of the records would allow the public to put the pieces together and see that we lost due to sabotage.