Halberstam, Sheehan, and Karnow would play crucial roles in events that fomented the coup that removed Diem on Nov. 1, 1963. Their anti-Diem information, much of it from ill-informed or agenda-driven sources, gave Diem's opponents in the US government the reasons they needed to remove what they considered to be an ineffective allied government. Ambassador Henry Cabot Lodge accepted their reports, spurring him to incite the coup.
They don't mention that their pet president, JFK, gave the OK on the Diem hit. Rumor has it that when Diem was already a prisoner in the back of a paddy wagon ( or some such) he called Lodge, trying to get off the hook. Lodge had already gotten the wink and the nod from Jack and Bobby and his reply to Diem was, "Sorry, Mr President, but I can't help you."
I believe that Diem and his brother were to be kidnapped. The execution was someone else's idea. In November of 1963, JFK was executed.